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

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Message-ID: <20210312101757.sqeyledbwjnpqdoy@ti.com>
Date:   Fri, 12 Mar 2021 15:47:59 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     <Tudor.Ambarus@...rochip.com>
CC:     <nm@...com>, <kristo@...nel.org>, <robh+dt@...nel.org>,
        <michael@...le.cc>, <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

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.

> 
> > 
> > Since PHY reads only work at an address that is 16-byte aligned and of
> > size that is a multiple of 16 bytes, read the starting and ending
> > unaligned portions without PHY, and only enable PHY for the middle part.
> > 
> > Signed-off-by: Pratyush Yadav <p.yadav@...com>
> > ---
> >  drivers/spi/spi-cadence-quadspi.c | 203 ++++++++++++++++++++++++++----
> >  1 file changed, 182 insertions(+), 21 deletions(-)
> > 

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.