Re: [PATCH RFC 4/4] mmc: host: renesas_sdhi_sys_dmac: Set dma_buswidth value to 32 byte

From: Wolfram Sang <wsa@the-dreams.de>
To: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: "linux-renesas-soc@vger.kernel.org"  <linux-renesas-soc@vger.kernel.org>
Subject: Re: [PATCH RFC 4/4] mmc: host: renesas_sdhi_sys_dmac: Set dma_buswidth value to 32 byte
Date: Mon, 2 Dec 2019 09:54:48 +0100	[thread overview]
Message-ID: <20191202085448.GD1266@kunai> (raw)
In-Reply-To: <TYAPR01MB45448366F6EB1F581CD399F7D8430@TYAPR01MB4544.jpnprd01.prod.outlook.com>

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Hi Shimoda-san,

> > 1) can't we set dma_priv->dma_buswidth at runtime when we know what the
> > card is capable of? Either DMA_SLAVE_BUSWIDTH_32_BYTES or
> > DMA_SLAVE_BUSWIDTH_4_BYTES? Then we don't need to fallback to PIO.
> > AFAIS, we only Gen2 sets .dma_buswidth in of_data, so we could even
> > remove it from of_data entirely?
> 
> As I replied to Ulrich-san on other email thread, for now, rcar-dmac has a limitation
> on dmaengine_slave_config(), we should not call it at runtime. But, I don't think
> any sd card have such a limitation. In other words, if rcar-dmac doesn't have
> the limitation, I think we can change the buswidth at runtime and then we can
> remove the .dma_buswidth from of_data.

So, that I understand correctly: The DMAC limitation is because of the
driver and not because of the HW? If so, is it hard/planned to be fixed?

> I also grepped in drivers/dma, and all dmaengine drivers except Renesas related
> SoCs don't support DMA_SLAVE_BUSWIDTH_32_BYTES. So, I think no driver uses
> the 32 bytes on mmc/hosts :)

Wow, we are bleeding edge with this? :)

Thanks,

   Wolfram

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