linux-kernel - Re: [PATCH 2/3] mmc: sdhci: sdhci-pci-o2micro: Check if controller supports 8-bit width

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Date:   Wed, 12 Jun 2019 09:08:32 -0600
From:   Raul Rangel <rrangel@...omium.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     linux-mmc@...r.kernel.org, ernest.zhang@...hubtech.com,
        djkurtz@...omium.org, linux-kernel@...r.kernel.org,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH 2/3] mmc: sdhci: sdhci-pci-o2micro: Check if controller
 supports 8-bit width

On Wed, Jun 12, 2019 at 04:09:47PM +0300, Adrian Hunter wrote:
> On 10/06/19 9:53 PM, Raul E Rangel wrote:
> > The O2 controller supports 8-bit EMMC access. mmc_select_bus_width()
> > will be used to determine if the MMC supports 8-bit or 4-bit access.
> 
> The problem is that the bit indicates a host controller capability, not how
> many data lines there actually are on the board.  Will this break something
> that does not have 8 lines?

So I asked the controller vendor about that:
> The capability shows the host controller can support 1,4,and 8 bit bus
> data transfer but it also depends on if HW can support it. Driver or FW
> should implement the bus testing procedure that is defined in A.6.3.a
> in JESD84-B51 spec to decide the real bus width that is supported in HW.

This seems to be what `mmc_select_bus_width()` is doing.

I don't actually have any 4-bit hardware to test with though.

Thanks for the review!