{"id":8010,"date":"2022-03-02T08:54:00","date_gmt":"2022-03-02T08:54:00","guid":{"rendered":"http:\/\/fastbitlab.com\/?p=8010"},"modified":"2023-09-15T12:20:14","modified_gmt":"2023-09-15T06:50:14","slug":"linux-device-driver-programming-lecture-41-read-method-implementation","status":"publish","type":"post","link":"https:\/\/fastbitlab.com\/blog\/linux-device-driver-programming-lecture-41-read-method-implementation\/","title":{"rendered":"Linux Device Driver Programming Lecture 41- Read method implementation"},"content":{"rendered":"
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Read method implementation<\/b><\/span><\/h1>\n
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 <\/p>\n<\/div>\n<\/div>\n<\/div>\n

What is the Read Method?<\/strong> <\/span><\/p>\n

The read method is a function that allows user-level processes to read data from a file or device managed by the kernel. It is commonly used for reading data from character devices, such as hardware peripherals or virtual files in the \/proc and \/sys filesystems.<\/span><\/p>\n

Parameters of the Read Method:<\/strong> <\/span>The read method typically takes the following parameters:<\/span><\/p>\n

    \n
  1. struct file *filp<\/code>: This parameter represents the file structure associated with the file or device you want to read from.<\/span><\/li>\n
  2. char __user *buff<\/code>: This is the user-space buffer where the data will be copied after reading.<\/span><\/li>\n
  3. size_t count<\/code>: The number of bytes to read.<\/span><\/li>\n
  4. loff_t *f_pos<\/code>: A pointer to the current file position. It keeps track of where the next read operation should start.<\/span><\/li>\n<\/ol>\n

     <\/p>\n

    How the Read Method Works:<\/strong> <\/span><\/p>\n

    Here’s a high-level overview of how the read method works:<\/span><\/p>\n

    In this article, let’s implement the read method. <\/span><\/p>\n

    Three important things you should be doing here. <\/span><\/p>\n

    One is you have to adjust the count, copy_to_user, after that, update the current file position. And after that, you have to return the appropriate information.<\/span><\/p>\n

    loff_t pcd_lseek(struct file *filp, loff_t offset, int whence)\r\n{\r\n    pr_info(\"lseek requested \\n\");\r\n    return<\/span> 0;\r\n\r\nssize_t pcd_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos)\r\n{\r\n    pr_info(\"Read requested for %zu bytes \\n\",count);\r\n\r\n   \/* Adjust the 'count' *\/<\/span>\r\n   if<\/span>((*f_pos + count) > DEV_MEM_SIZE)\r\n       count = DEV_MEM_SIZE - *f_pos;\r\n\r\n\/*copy to user *\/<\/span>\r\n\r\n}\r\n\r\n\/*update the current file postion *\/<\/span> \r\n\r\nreturn<\/span> 0;\r\n\r\n}\r\n\r\n<\/pre>\n
    Adjust the count in read method<\/span><\/p>\n
    Adjusting the ‘Count’ Parameter:<\/strong><\/span><\/p>\n
    If the current file position, how do you get  the current file position by dereferencing this(*f_pos). Dereferencing of this pointer. Current file position + count.<\/span><\/p>\n
    If this(*f_pos + count) exceeds our maximum size of the device memory, that is DEV_MEM_SIZE, then you should trim the count value. Count will become count =DEV_MEM_SIZE minus the current file position. That’s how we limit the value of count.<\/span><\/p>\n
    Let’s say, initially, I said the file position is 0. Now, let’s say the count requested is 600 bytes. 600 is greater than 512. That’s why count will become 512 – 0.<\/span><\/p>\n
    And after that, copy_to_user. Here, we should use copy_to_user kernel function. First is always destination.<\/span><\/p>\n
     What is a destination? <\/span><\/p>\n
    This is destination user buffer. Buffer from the kernel space memory that is this pointer device_buffer from current file position. That’s why we have to use the current file position value. <\/span><\/p>\n
    From here, the requested number of bytes that is count. This could return non-zero values. That’s why you have to check the return value.<\/span><\/p>\n
    If this returns non-zero, then you should return -EFAULT, that is bad address.<\/span><\/p>\n
    \/*copy to user *\/<\/span>\r\nif<\/span>(copy_to_user(buff, device_buffer[*f_pos],count)){\r\n       return<\/span> -EFAULT;\r\n\r\n}<\/pre>\n
    Copy to user<\/span><\/p>\n
    And never use a single line commenting, that is a bad practice as per kernel programming guideline is concerned, always use multiline commenting.<\/span><\/p>\n
    After that, you have to update the current file position * of current file position += count. And then, you should be returning the number of  bytes which have been successfully read. That is, you should be return count.<\/span><\/p>\n
    \/*update the current file postion *\/<\/span>\r\n*f_pos += count;\r\npr_info(\"Number of bytes successfully read = %zu\\n\",count);\r\npr_info(\"Updated file position = %lld\\n\",*f_pos);<\/pre>\n
    Update the current file position<\/span><\/p>\n
    Let’s also print some messages. The above message(<\/span>pr_info(\u201cRead requested for %zu bytes \\n\u201d,count);<\/b>) actually prints the actual count requested. What we can do is we can put one more print statement. Pr_info “number of bytes successfully read =%zu\\n”, and let’s print count.<\/span><\/span><\/p>\n
    And also, let’s print the updated value of the file position. <\/span><\/span><\/p>\n
    Updated file position = what you should be using is, you should print this value(*f_pos). The data type is long offset. This is actually typedef for long, long int. This is signed data.  <\/span>So, for that, you should be using %lld. <\/span><\/span><\/p>\n
    As per the kernel documentation, you should use this format specifier for long long int \\n. Let’s print this value  * of file position.<\/span><\/span><\/p>\n
    And after that, let’s also print, the initial value of the file position. I would write And after that, let’s also print the initial value of the file position. I would write the current file position. That means, before reading. Current file position = %lld\\n, file position That’s about read, and in the next article, let’s implement write, and after that, we will test this.<\/span><\/p>\n
     <\/p>\n
    FastBit Embedded Brain Academy Courses<\/b><\/span><\/p>\n
    Click here:  <\/span><\/span>https:\/\/fastbitlab.com\/course1<\/span><\/a><\/span><\/p>\n
     <\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
      Read method implementation     What is the Read Method? The read method is a function that allows user-level processes to read data from a file or device managed by the kernel. It is commonly used for reading data from character devices, such as hardware peripherals or virtual files in the \/proc and \/sys […]<\/p>\n","protected":false},"author":1,"featured_media":8014,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ocean_post_layout":"","ocean_both_sidebars_style":"","ocean_both_sidebars_content_width":0,"ocean_both_sidebars_sidebars_width":0,"ocean_sidebar":"0","ocean_second_sidebar":"0","ocean_disable_margins":"enable","ocean_add_body_class":"","ocean_shortcode_before_top_bar":"","ocean_shortcode_after_top_bar":"","ocean_shortcode_before_header":"","ocean_shortcode_after_header":"","ocean_has_shortcode":"","ocean_shortcode_after_title":"","ocean_shortcode_before_footer_widgets":"","ocean_shortcode_after_footer_widgets":"","ocean_shortcode_before_footer_bottom":"","ocean_shortcode_after_footer_bottom":"","ocean_display_top_bar":"default","ocean_display_header":"default","ocean_header_style":"","ocean_center_header_left_menu":"0","ocean_custom_header_template":"0","ocean_custom_logo":0,"ocean_custom_retina_logo":0,"ocean_custom_logo_max_width":0,"ocean_custom_logo_tablet_max_width":0,"ocean_custom_logo_mobile_max_width":0,"ocean_custom_logo_max_height":0,"ocean_custom_logo_tablet_max_height":0,"ocean_custom_logo_mobile_max_height":0,"ocean_header_custom_menu":"0","ocean_menu_typo_font_family":"0","ocean_menu_typo_font_subset":"","ocean_menu_typo_font_size":0,"ocean_menu_typo_font_size_tablet":0,"ocean_menu_typo_font_size_mobile":0,"ocean_menu_typo_font_size_unit":"px","ocean_menu_typo_font_weight":"","ocean_menu_typo_font_weight_tablet":"","ocean_menu_typo_font_weight_mobile":"","ocean_menu_typo_transform":"","ocean_menu_typo_transform_tablet":"","ocean_menu_typo_transform_mobile":"","ocean_menu_typo_line_height":0,"ocean_menu_typo_line_height_tablet":0,"ocean_menu_typo_line_height_mobile":0,"ocean_menu_typo_line_height_unit":"","ocean_menu_typo_spacing":0,"ocean_menu_typo_spacing_tablet":0,"ocean_menu_typo_spacing_mobile":0,"ocean_menu_typo_spacing_unit":"","ocean_menu_link_color":"","ocean_menu_link_color_hover":"","ocean_menu_link_color_active":"","ocean_menu_link_background":"","ocean_menu_link_hover_background":"","ocean_menu_link_active_background":"","ocean_menu_social_links_bg":"","ocean_menu_social_hover_links_bg":"","ocean_menu_social_links_color":"","ocean_menu_social_hover_links_color":"","ocean_disable_title":"default","ocean_disable_heading":"default","ocean_post_title":"","ocean_post_subheading":"","ocean_post_title_style":"","ocean_post_title_background_color":"","ocean_post_title_background":0,"ocean_post_title_bg_image_position":"","ocean_post_title_bg_image_attachment":"","ocean_post_title_bg_image_repeat":"","ocean_post_title_bg_image_size":"","ocean_post_title_height":0,"ocean_post_title_bg_overlay":0.5,"ocean_post_title_bg_overlay_color":"","ocean_disable_breadcrumbs":"default","ocean_breadcrumbs_color":"","ocean_breadcrumbs_separator_color":"","ocean_breadcrumbs_links_color":"","ocean_breadcrumbs_links_hover_color":"","ocean_display_footer_widgets":"default","ocean_display_footer_bottom":"default","ocean_custom_footer_template":"0","ocean_post_oembed":"","ocean_post_self_hosted_media":"","ocean_post_video_embed":"","ocean_link_format":"","ocean_link_format_target":"self","ocean_quote_format":"","ocean_quote_format_link":"post","ocean_gallery_link_images":"off","ocean_gallery_id":[],"footnotes":""},"categories":[8],"tags":[18],"class_list":["post-8010","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blog","tag-linux-device-driver-programming","entry","has-media"],"yoast_head":"\nRead method | Linux Device Driver Programming<\/title>\n<meta name=\"description\" content=\"Learn how to implement the read method in kernel programming, including adjusting the count and updating the file position. 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