Struggling to get kernel to preserve framebuffer memory when booting

Dushara Jayasinghe <dushara@xxxxxxxxxxxxxxxxx> · Wed, 6 Dec 2017 09:58:00 +1100



    Hi all,
    I'm struggling to solve a problem which in theory should be quite
      straight forward. This email is a bit of a brain dump so apologies
      for being all over the place.

    
    I'm working on an embedded device where u-boot displays a logo
      which I want the kernel to preserve. However I find the display
      gets blanked as soon as the kernel starts executing.
    As I understand it, the first step is to get the kernel to
      reserve the frame-buffer memory (and hand it to the FB driver
      later on). So I added the following device tree entry:
            reserved-memory {

                      #address-cells = <1>;

                      #size-cells = <1>;

                      ranges;

      
                      display_reserved: fb@0 {

                              no-map;

                              reg = <0x3ffa5000
        0x0005b000>;

                      };

              };

      
    The memory area reflects the FB memory allocated by u-boot.
    This is an atmel SAMA5D2 based board with 512M RAM in the
      0x20000000-0x40000000 range.

    
    However, I find the display is blanked even before the FB driver
    starts.
    I have confirmed that the device tree entry is processed by
      placing printks in drivers/of/of_reserved_mem.c
(http://elixir.free-electrons.com/linux/latest/source/drivers/of/of_reserved_mem.c#L85).
    I also added the following line at the same spot to see if that
      area is at least accessible:
    *(unsigned int*)0x3ffa5000  = 0xffffffff
    And I observed the following result:
    
      The kernel hangs
      The display is not yet blanked at this point
    
    Writing to the same location from within the u-boot shell places
      a white pixel on the display.
    Could anyone suggest what areas I need to look at to solve this
      problem? Any tips and suggestions will be greatly appreciated.
    Thank you
    Dushara

    
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