linux-kernel - Re: [RFC PATCH 4/6] spi: cadence-qspi: Use PHY for DAC reads if possible

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Message-ID: <ee2b753b16e76ecbede4c1373b6f2d77@walle.cc>
Date:   Thu, 29 Apr 2021 18:28:13 +0200
From:   Michael Walle <michael@...le.cc>
To:     Pratyush Yadav <p.yadav@...com>
Cc:     Tudor.Ambarus@...rochip.com, nm@...com, kristo@...nel.org,
        robh+dt@...nel.org, miquel.raynal@...tlin.com, richard@....at,
        vigneshr@...com, broonie@...nel.org,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-spi@...r.kernel.org, lokeshvutla@...com
Subject: Re: [RFC PATCH 4/6] spi: cadence-qspi: Use PHY for DAC reads if
 possible

Am 2021-03-12 11:17, schrieb Pratyush Yadav:
> On 12/03/21 09:13AM, Tudor.Ambarus@...rochip.com wrote:
>> On 3/11/21 9:12 PM, Pratyush Yadav wrote:
>> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>> >
>> > Check if a read is eligible for PHY and if it is, enable PHY and DQS.
>> 
>> DQS as in data strobe? Shouldn't the upper layer inform the QSPI 
>> controller
>> whether DS is required or not?
> 
> Yes, DQS as in data strobe. I need to check this again, but IIRC the
> controller cannot run in PHY mode unless DS is used. Ideally the upper
> layer should indeed inform the controller whether DS is 
> supported/in-use
> or not. That can be used to decide whether PHY mode (and consequently
> the DS line) is to be used or not.
> 
> Currently there are only two flashes that use 8D-8D-8D mode (S28HS512T
> and MT35XU512ABA), and both of them drive the DS line.

The LS1028A datasheet explicitly states that the calibration is only
used for non-DQS flashes. Which makes sense, because it just determine 
at
which point the input data is sampled. And if the flash provides a data
strobe, it already know when to sample it. What I am missing here?

-michael