[PATCH v2 0/5] drm: Reuse temporary memory for format conversion

[Jocelyn Falempe]

On 20/09/2023 16:24, Thomas Zimmermann wrote:
> DRM's format-conversion helpers require temporary memory. Pass the
> buffer from the caller and keep it allocated over several calls. Allow
> the caller to preallocate the buffer memory.
> 
> The motivation for this patchset is the recent work on a DRM panic
> handler. The panic handler requires format conversion to display an
> error to the screen. But allocating memory during kernel panics is
> fragile. The changes in this patchset enable the DRM panic handler to
> preallocate buffer storage before the panic occurs.
> 
> As an additonal benefit, drivers can now keep the temporary storage
> across multiple display updates. Avoiding memory allocation reduces
> the CPU overhead of the format helpers.
> 
> Patch 1 adds struct drm_xfrm_buf, a simple interface to pass around
> the buffer storage. Patch 2 moves the memory management from the format
> helpers into their callers. Drivers release the temporary storage at
> the end of their display-update functions.
> 
> Patches 3 to 8 update three drivers to keep the allocated memory for
> all of a device's lifetime. Managed cleanup releases the buffer as part
> of releaseing the device. As additional benefit, buffer allocation now
> happens in atomic_check helpers. The driver thus detects OOM errors
> before the display update begins.

Thanks for the patches.
I'm still experimenting with drm_panic, and it's not clear if it will 
use that or not yet.
But it's already useful for other drivers, avoiding alloc/free for each 
frame, is a good thing.


Best regards,

-- 

Jocelyn


> 
> Tested with simpledrm.
> 
> v2:
> 	* reserve storage during probing in the drivers
> 
> Thomas Zimmermann (5):
>    drm/format-helper: Add struct drm_xfrm_buf to cache format conversion
>    drm/format-helper: Pass xfrm buffer to format-conversion helpers
>    drm/simpledrm: Store xfrm buffer in device instance
>    drm/ofdrm: Store xfrm buffer in device instance
>    drm/ssd130x: Store xfrm buffer in device instance
> 
>   drivers/gpu/drm/drm_format_helper.c           | 204 +++++++++++++-----
>   drivers/gpu/drm/drm_mipi_dbi.c                |   7 +-
>   drivers/gpu/drm/gud/gud_pipe.c                |  21 +-
>   drivers/gpu/drm/solomon/ssd130x.c             |  16 +-
>   drivers/gpu/drm/solomon/ssd130x.h             |   3 +
>   .../gpu/drm/tests/drm_format_helper_test.c    |  33 +--
>   drivers/gpu/drm/tiny/cirrus.c                 |   5 +-
>   drivers/gpu/drm/tiny/ofdrm.c                  |  11 +-
>   drivers/gpu/drm/tiny/repaper.c                |   5 +-
>   drivers/gpu/drm/tiny/simpledrm.c              |  11 +-
>   drivers/gpu/drm/tiny/st7586.c                 |   5 +-
>   include/drm/drm_format_helper.h               |  74 +++++--
>   12 files changed, 300 insertions(+), 95 deletions(-)
>