Re: [PATCH 04/10] drm/tegra: Set fbdev flags

Hi Javier

Am 05.07.23 um 10:34 schrieb Javier Martinez Canillas:

Thomas Zimmermann [off-list ref] writes:

quoted

Set fbdev default flags FBNFO_DEFAULT and mark the framebuffer with
FBINFO_VIRTFB. The framebuffer range is in DMA-able memory and should
be accessed with the CPU's regular memory ops.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thierry Reding <redacted>
Cc: Mikko Perttunen <mperttunen@nvidia.com>
---
  drivers/gpu/drm/tegra/fbdev.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/tegra/fbdev.c b/drivers/gpu/drm/tegra/fbdev.c
index 82577b7c88da..8074430c52f1 100644
--- a/drivers/gpu/drm/tegra/fbdev.c
+++ b/drivers/gpu/drm/tegra/fbdev.c

@@ -103,6 +103,8 @@ static int tegra_fbdev_probe(struct drm_fb_helper *helper,
  		return PTR_ERR(info);
  	}
  
+	info->flags = FBINFO_DEFAULT;
+
  	fb = tegra_fb_alloc(drm, &cmd, &bo, 1);
  	if (IS_ERR(fb)) {
  		err = PTR_ERR(fb);

@@ -132,6 +134,7 @@ static int tegra_fbdev_probe(struct drm_fb_helper *helper,
  		}
  	}
  
+	info->flags |= FBINFO_VIRTFB;

I see that all fbdev drivers just do: info->flags = FBINFO_FLAG_DEFAULT | FBINFO_VIRTFB

Guess you are doing in two assignments to be consistent with drm_fbdev_dma.c ?
I was just curious about the rationale for setting the flags in two steps.

The _DEFAULT flag is really just a zero. And the other flags describe different aspects of the framebuffer.  I think it makes sense to set the flags together with the respective state. For example, _VIRTFB is set next to ->screen_buffer, because they belong together.

_VIRTFB is currently only used in defio code at

https://elixir.bootlin.com/linux/latest/source/drivers/video/fbdev/core/fb_defio.c#L232

I think the fbdev I/O helpers should also test this flag after all drivers have been annotated correctly. For example, fb_io_read() would WARN_ONCE if the _VIRTFB flag has been set; and fb_sys_read() would warn if it hasn't been set.  For the read helpers, it also makes sense to WARN_ONCE if the _READS_FAST flag has not been set.

Best regards
Thomas

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
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