linux-kernel - Re: [RESEND PATCH v2 3/5] drm/msm: call dma_direct

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Date:   Sat, 14 Nov 2020 17:21:59 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jonathan Marek <jonathan@...ek.ca>
Cc:     freedreno@...ts.freedesktop.org, hch@....de,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v2 3/5] drm/msm: call dma_direct_bypass()

On Sat, Nov 14, 2020 at 10:17:11AM -0500, Jonathan Marek wrote:
> Always use direct dma ops and no swiotlb.
> 
> Note: arm-smmu-qcom already avoids creating iommu dma ops, but not
> everything uses arm-smmu-qcom and this also sets the dma mask.
> 
> Signed-off-by: Jonathan Marek <jonathan@...ek.ca>
> ---
>  drivers/gpu/drm/msm/Kconfig   | 1 +
>  drivers/gpu/drm/msm/msm_drv.c | 8 +++++---
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/Kconfig b/drivers/gpu/drm/msm/Kconfig
> index e5816b498494..07c50405970a 100644
> --- a/drivers/gpu/drm/msm/Kconfig
> +++ b/drivers/gpu/drm/msm/Kconfig
> @@ -20,6 +20,7 @@ config DRM_MSM
>  	select SND_SOC_HDMI_CODEC if SND_SOC
>  	select SYNC_FILE
>  	select PM_OPP
> +	select DMA_OPS_BYPASS
>  	help
>  	  DRM/KMS driver for MSM/snapdragon.
>  
> diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> index 49685571dc0e..bae48afca82e 100644
> --- a/drivers/gpu/drm/msm/msm_drv.c
> +++ b/drivers/gpu/drm/msm/msm_drv.c
> @@ -6,6 +6,7 @@
>   */
>  
>  #include <linux/dma-mapping.h>
> +#include <linux/dma-direct.h>
>  #include <linux/kthread.h>
>  #include <linux/uaccess.h>
>  #include <uapi/linux/sched/types.h>
> @@ -1288,10 +1289,11 @@ static int msm_pdev_probe(struct platform_device *pdev)
>  	if (ret)
>  		goto fail;
>  
> -	/* on all devices that I am aware of, iommu's which can map
> -	 * any address the cpu can see are used:
> +	/* always use direct dma ops and no swiotlb

Again, and implementation detail.  Comments should not explain details
obvious from the code (especially layers away) but the intent.