[PATCH] drm/msm: use proper memory barriers for updating tail/head

[Daniel Vetter]

On Mon, Oct 02, 2017 at 10:37:11AM -0400, Rob Clark wrote:
> Fixes intermittent corruption of cmdstream dump.
> 
> Signed-off-by: Rob Clark <robdclark at gmail.com>

Rule of memory barriers: They always need to come in pairs, and you should
put a comment to each one explaining where the other side is. I.e. for all
the store_release ones you add here you need to put a few read_acquire
ones somewhere else, and then explain how they link.

One side only doesn't really work with barriers - except when you're lucky
and on x86 :-)
-Daniel
> ---
>  drivers/gpu/drm/msm/msm_rd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_rd.c b/drivers/gpu/drm/msm/msm_rd.c
> index 5c087a4228ff..918c65e19964 100644
> --- a/drivers/gpu/drm/msm/msm_rd.c
> +++ b/drivers/gpu/drm/msm/msm_rd.c
> @@ -120,7 +120,7 @@ static void rd_write(struct msm_rd_state *rd, const void *buf, int sz)
>  		n = min(sz, circ_space_to_end(&rd->fifo));
>  		memcpy(fptr, ptr, n);
>  
> -		fifo->head = (fifo->head + n) & (BUF_SZ - 1);
> +		smp_store_release(&fifo->head, (fifo->head + n) & (BUF_SZ - 1));
>  		sz  -= n;
>  		ptr += n;
>  
> @@ -157,7 +157,7 @@ static ssize_t rd_read(struct file *file, char __user *buf,
>  		goto out;
>  	}
>  
> -	fifo->tail = (fifo->tail + n) & (BUF_SZ - 1);
> +	smp_store_release(&fifo->tail, (fifo->tail + n) & (BUF_SZ - 1));
>  	*ppos += n;
>  
>  	wake_up_all(&rd->fifo_event);
> -- 
> 2.13.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch