linux-kernel - Re: [PATCH 05/12] bcm2835-dma: Derive slave DMA addresses correctly

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Message-ID: <ee19a95d-fe1e-4f3f-bc81-bdef38475469@arm.com>
Date: Mon, 5 Feb 2024 18:03:53 +0000
From: Robin Murphy <robin.murphy@....com>
To: Andrea della Porta <andrea.porta@...e.com>, Vinod Koul
 <vkoul@...nel.org>, Florian Fainelli <florian.fainelli@...adcom.com>,
 Ray Jui <rjui@...adcom.com>, Scott Branden <sbranden@...adcom.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, dmaengine@...r.kernel.org,
 linux-rpi-kernel@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Cc: Maxime Ripard <maxime@...no.tech>, Dom Cobley <popcornmix@...il.com>,
 Phil Elwell <phil@...pberrypi.com>
Subject: Re: [PATCH 05/12] bcm2835-dma: Derive slave DMA addresses correctly

On 2024-02-04 6:59 am, Andrea della Porta wrote:
> From: Phil Elwell <phil@...pberrypi.com>
> 
> Slave addresses for DMA are meant to be supplied as physical addresses
> (contrary to what struct snd_dmaengine_dai_dma_data does). It is up to
> the DMA controller driver to perform the translation based on its own
> view of the world, as described in Device Tree.
> 
> Now that the Pi Device Trees have the correct peripheral mappings,
> replace the hacky address munging with phys_to_dma().
> 
> Signed-off-by: Phil Elwell <phil@...pberrypi.com>
> ---
>   drivers/dma/bcm2835-dma.c | 23 +++++------------------
>   1 file changed, 5 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> index 237dcdb8d726..077812eda609 100644
> --- a/drivers/dma/bcm2835-dma.c
> +++ b/drivers/dma/bcm2835-dma.c
> @@ -18,6 +18,7 @@
>    *	Copyright 2012 Marvell International Ltd.
>    */
>   #include <linux/dmaengine.h>
> +#include <linux/dma-direct.h>

Please read the comment at the top of that file; this driver is 
definitely not a DMA API implementation, and should not be including it.

>   #include <linux/dma-mapping.h>
>   #include <linux/dmapool.h>
>   #include <linux/err.h>
> @@ -980,22 +981,12 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_slave_sg(
>   	if (direction == DMA_DEV_TO_MEM) {
>   		if (c->cfg.src_addr_width != DMA_SLAVE_BUSWIDTH_4_BYTES)
>   			return NULL;
> -		src = c->cfg.src_addr;
> -		/*
> -		 * One would think it ought to be possible to get the physical
> -		 * to dma address mapping information from the dma-ranges DT
> -		 * property, but I've not found a way yet that doesn't involve
> -		 * open-coding the whole thing.
> -		 */
> -		if (c->is_40bit_channel)
> -			src |= 0x400000000ull;
> +		src = phys_to_dma(chan->device->dev, c->cfg.src_addr);

FWIW I'd argue that abusing DMA API internals like this is even more 
hacky than bypassing it entirely. The appropriate public API for setting 
up the device end of a transfer is dma_map_resource(). Now, it *is* the 
case currently that the dma-direct implementation of that does not take 
dma_range_map into account, but that's already an open question:

https://lore.kernel.org/linux-iommu/20220610080802.11147-1-Sergey.Semin@baikalelectronics.ru/

Thanks,
Robin.

>   		info |= BCM2835_DMA_S_DREQ | BCM2835_DMA_D_INC;
>   	} else {
>   		if (c->cfg.dst_addr_width != DMA_SLAVE_BUSWIDTH_4_BYTES)
>   			return NULL;
> -		dst = c->cfg.dst_addr;
> -		if (c->is_40bit_channel)
> -			dst |= 0x400000000ull;
> +		dst = phys_to_dma(chan->device->dev, c->cfg.dst_addr);
>   		info |= BCM2835_DMA_D_DREQ | BCM2835_DMA_S_INC;
>   	}
>   
> @@ -1064,17 +1055,13 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_cyclic(
>   	if (direction == DMA_DEV_TO_MEM) {
>   		if (c->cfg.src_addr_width != DMA_SLAVE_BUSWIDTH_4_BYTES)
>   			return NULL;
> -		src = c->cfg.src_addr;
> -		if (c->is_40bit_channel)
> -			src |= 0x400000000ull;
> +		src = phys_to_dma(chan->device->dev, c->cfg.src_addr);
>   		dst = buf_addr;
>   		info |= BCM2835_DMA_S_DREQ | BCM2835_DMA_D_INC;
>   	} else {
>   		if (c->cfg.dst_addr_width != DMA_SLAVE_BUSWIDTH_4_BYTES)
>   			return NULL;
> -		dst = c->cfg.dst_addr;
> -		if (c->is_40bit_channel)
> -			dst |= 0x400000000ull;
> +		dst = phys_to_dma(chan->device->dev, c->cfg.dst_addr);
>   		src = buf_addr;
>   		info |= BCM2835_DMA_D_DREQ | BCM2835_DMA_S_INC;
>