linux-kernel - Re: [PATCH] drm/ttm: only use DMA32 if needed for dummy_read

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Date: Tue, 16 Jan 2024 10:45:46 +0100
From: Christian König <christian.koenig@....com>
To: Yangyu Chen <cyy@...self.name>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Cc: Huang Rui <ray.huang@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Jiuyang Liu <liu@...yang.me>, Yichuan Gao <i@...is.me>
Subject: Re: [PATCH] drm/ttm: only use DMA32 if needed for dummy_read_page

Am 16.01.24 um 08:24 schrieb Yangyu Chen:
> Some platforms may not have any memory in ZONE_DMA32 and use IOMMU to allow
> 32-bit-DMA-only device to work. Forcing GFP_DMA32 on dummy_read_page will
> fail in such platforms. Only use DMA32 when it must to get the bug
> resolved.

Well that makes no sense.

If a platform doesn't have a ZONE_DMA32 then GFP_DMA32 is just ignored 
as far as I know.

This patch here won't work since the use_dma32 flag is a per device flag 
which can't be used for the global initialization.

Otherwise this can randomly fail depending on if a DMA32 device 
initializes first or after some device with larger addressing 
capabilities. This configuration is quite common on older motherboards 
with both integrated and dedicated graphics.

Regards,
Christian.

>
> Signed-off-by: Yangyu Chen <cyy@...self.name>
> ---
>   drivers/gpu/drm/ttm/ttm_device.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_device.c b/drivers/gpu/drm/ttm/ttm_device.c
> index d48b39132b32..62f16fb72428 100644
> --- a/drivers/gpu/drm/ttm/ttm_device.c
> +++ b/drivers/gpu/drm/ttm/ttm_device.c
> @@ -63,7 +63,7 @@ static void ttm_global_release(void)
>   	mutex_unlock(&ttm_global_mutex);
>   }
>   
> -static int ttm_global_init(void)
> +static int ttm_global_init(bool use_dma32)
>   {
>   	struct ttm_global *glob = &ttm_glob;
>   	unsigned long num_pages, num_dma32;
> @@ -95,7 +95,8 @@ static int ttm_global_init(void)
>   	ttm_pool_mgr_init(num_pages);
>   	ttm_tt_mgr_init(num_pages, num_dma32);
>   
> -	glob->dummy_read_page = alloc_page(__GFP_ZERO | GFP_DMA32);
> +	glob->dummy_read_page = use_dma32 ? alloc_page(__GFP_ZERO | GFP_DMA32) :
> +					    alloc_page(__GFP_ZERO);
>   
>   	if (unlikely(glob->dummy_read_page == NULL)) {
>   		ret = -ENOMEM;
> @@ -200,7 +201,7 @@ int ttm_device_init(struct ttm_device *bdev, const struct ttm_device_funcs *func
>   	if (WARN_ON(vma_manager == NULL))
>   		return -EINVAL;
>   
> -	ret = ttm_global_init();
> +	ret = ttm_global_init(use_dma32);
>   	if (ret)
>   		return ret;
>