Re: [PATCH] drm: Add drm_mode_display_size helper function

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Sep 30, 2025 at 09:26:55PM +0000, Jonathan Cavitt wrote:
> Add a helper function that computes the product of hdisplay and
> vdisplay.
> 
> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/gpu/drm/drm_gem_vram_helper.c | 2 +-
>  drivers/gpu/drm/drm_mipi_dbi.c        | 2 +-
>  include/drm/drm_modes.h               | 5 +++++
>  3 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_vram_helper.c b/drivers/gpu/drm/drm_gem_vram_helper.c
> index 90760d0ca071..4ac71c34f4c6 100644
> --- a/drivers/gpu/drm/drm_gem_vram_helper.c
> +++ b/drivers/gpu/drm/drm_gem_vram_helper.c
> @@ -967,7 +967,7 @@ drm_vram_helper_mode_valid_internal(struct drm_device *dev,
>  
>  	max_fbpages = (vmm->vram_size / 2) >> PAGE_SHIFT;
>  
> -	fbsize = mode->hdisplay * mode->vdisplay * max_bpp;

This calculation is quite inaccurate since it doesn't
consider any stride alignment requirements. Maybe it works
for a few cases, but certainly not usable on most hardware.

> +	fbsize = drm_mode_display_size(mode) * max_bpp;
>  	fbpages = DIV_ROUND_UP(fbsize, PAGE_SIZE);
>  
>  	if (fbpages > max_fbpages)
> diff --git a/drivers/gpu/drm/drm_mipi_dbi.c b/drivers/gpu/drm/drm_mipi_dbi.c
> index e33c78fc8fbd..54212bf85160 100644
> --- a/drivers/gpu/drm/drm_mipi_dbi.c
> +++ b/drivers/gpu/drm/drm_mipi_dbi.c
> @@ -691,7 +691,7 @@ int mipi_dbi_dev_init(struct mipi_dbi_dev *dbidev,
>  		      const struct drm_simple_display_pipe_funcs *funcs,
>  		      const struct drm_display_mode *mode, unsigned int rotation)
>  {
> -	size_t bufsize = mode->vdisplay * mode->hdisplay * sizeof(u16);
> +	size_t bufsize = drm_mode_display_size(mode) * sizeof(u16);
>  
>  	dbidev->drm.mode_config.preferred_depth = 16;
>  
> diff --git a/include/drm/drm_modes.h b/include/drm/drm_modes.h
> index b9bb92e4b029..775c94c55cda 100644
> --- a/include/drm/drm_modes.h
> +++ b/include/drm/drm_modes.h
> @@ -572,4 +572,9 @@ struct drm_display_mode *
>  drm_mode_create_from_cmdline_mode(struct drm_device *dev,
>  				  struct drm_cmdline_mode *cmd);
>  
> +static inline u32 drm_mode_display_size(const struct drm_display_mode *mode)

"display size" isn't really a term that gives me any kind of idea
what this does, so the name is rather poor. drm_mode_num_active_pixels()
or something might make more sense. But I'm not sure there's much
point in this helper since that value is very rarely useful for
anything.

> +{
> +	return mode->hdisplay * mode->vdisplay;
> +}
> +
>  #endif /* __DRM_MODES_H__ */
> -- 
> 2.43.0

-- 
Ville Syrjälä
Intel