linux-kernel - Re: [PATCH v2] mmc: tegra: Implement enable_dma() to set dma

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Date:   Thu, 15 Aug 2019 14:48:20 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>, ulf.hansson@...aro.org,
        thierry.reding@...il.com, jonathanh@...dia.com
Cc:     linux-mmc@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, vdumpa@...dia.com
Subject: Re: [PATCH v2] mmc: tegra: Implement enable_dma() to set dma_mask

On 14/08/19 3:57 AM, Nicolin Chen wrote:
> [ Integrated the change and commit message made by Thierry Reding ]
> 
> The SDHCI controller found in early Tegra SoCs (from Tegra20 through
> Tegra114) used an AHB interface to the memory controller, which allowed
> only 32 bits of memory to be addressed.
> 
> Starting with Tegra124, this limitation was removed by making the SDHCI
> controllers native MCCIF clients, which means that they got increased
> bandwidth and better arbitration to the memory controller as well as an
> address range extended to 40 bits, out of which only 34 were actually
> used (bits 34-39 are tied to 0 in the controller).
> 
> For Tegra186, all of the 40 bits can be used; For Tegra194, 39-bit can
> be used.
> 
> So far, sdhci-tegra driver has been relying on sdhci core to configure
> the DMA_BIT_MASK between 32-bit or 64-bit, using one of quirks2 flags:
> SDHCI_QUIRK2_BROKEN_64_BIT_DMA. However, there is a common way, being
> mentioned in sdhci.c file, to set dma_mask via enable_dma() callback in
> the device driver directly.
> 
> So this patch implements an enable_dma() callback in the sdhci-tegra,
> in order to set an accurate DMA_BIT_MASK, other than just 32/64 bits.

Is there a reason this cannot be done at probe time?