Re: dw_mmc: IDMAC Invalidate cache after read

[Robin Murphy <robin.murphy@arm.com>]

Hi Jan,

[repeating some of the discussion from your other thread for the benefit 
of the MMC audience]

On 21/11/2018 07:42, JABLONSKY Jan wrote:
> CPU may not see most up-to-date and correct copy of DMA buffer, when
> internal DMA controller is in use.
> Problem appears on The Altera SoC FPGA (uses integrated DMA controller),
> during higher CPU and system memory load
> 
> Signed-off-by: Jan Jablonsky <jan.jablonsky@thalesgroup.com>
> ---
>   drivers/mmc/host/dw_mmc.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> index 80dc2fd..63873d9 100644
> --- a/drivers/mmc/host/dw_mmc.c
> +++ b/drivers/mmc/host/dw_mmc.c
> @@ -499,8 +499,7 @@ static void dw_mci_dmac_complete_dma(void *arg)
>   
>   	dev_vdbg(host->dev, "DMA complete\n");
>   
> -	if ((host->use_dma == TRANS_MODE_EDMAC) &&
> -	    data && (data->flags & MMC_DATA_READ))
> +	if (data && (data->flags & MMC_DATA_READ))
>   		/* Invalidate cache after read */
>   		dma_sync_sg_for_cpu(mmc_dev(host->slot->mmc),
>   				    data->sg,

It looks very dubious whether this is actually the right thing to do. 
Just considering this driver, edma has an complementary sync_sg call in 
its .start method, so if idma needed this one, logically shouldn't it 
also need the other one as well?

However, from a DMA API point of view, these syncs make no sense either 
way - the very next thing we do here is call host->dma_ops->cleanup(), 
which calls dma_unmap_sg(), which will perform the appropriate cache 
maintenance anyway. Thus I can't see why this code is even here to begin 
with. Similarly on the request path - the sg list really shouldn't have 
been touched since being mapped in dw_mci_pre_dma_transfer(), so that 
sync should also be an effective no-op unless it's papering over some 
race condition elsewhere.

Shawn - do you remember why these syncs were added in 3fc7eaef44dbc? 
Were you seeing actual coherency issues on RK31xx SoCs, or was it 
perhaps just some leftover or misunderstanding which missed getting 
cleaned up?

Robin.