linux-kernel - Re: [PATCH] scsi: ufs: Consider device limitations for dma

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Message-ID: <20190114173659.GA17788@infradead.org>
Date:   Mon, 14 Jan 2019 09:36:59 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Vinayak Holikatti <vinholikatti@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH] scsi: ufs: Consider device limitations for dma_mask

On Mon, Jan 14, 2019 at 09:30:51AM -0800, Bjorn Andersson wrote:
> The problem here is that the capability bit states that the controller
> itself claim to be able to deal with 64-bit addresses, which is probably
> true. The thing that the struct device represents (the integrated
> controller, on a bus in this SoC) doesn't.
> 
> The device model accurately handles this and carries a dma_mask that's
> appropriate for the device in this system - the capability is not.
> 
> > You either need to introduce a quirk or a way to communicate the
> > different limit so that it can be set by the core.
> 
> The system's limit is already communicated in hba->dev->dma_mask, but
> the ufshcd driver overwrites this. I expect that this would make sense
> if the device model claims we can do e.g. 40 bit addressing, but the
> 64-bit capability is not set in the controller - in which case ufshcd
> would accurately lower this to 32-bits.

No, that is absolutely not true.  dev->dma_mask is set by the driver
to what the driver based on the device specsheet/register claims to
support.  dev->bus_dma_mask contains any additional limits imposed
by the bus/system, but that is handled transparently by the dma mapping
code.