Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <a92fdddcd2c75bc6c11ad0b2824bbac84db5b06d.camel@linux.ibm.com>
Subject: Re: [PATCH 0/2] spi: fsi: Reduce max transfer size to 8 bytes
From:   Eddie James <eajames@linux.ibm.com>
To:     David Laight <David.Laight@ACULAB.COM>,
        "linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>
Cc:     "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "broonie@kernel.org" <broonie@kernel.org>,
        "openbmc@lists.ozlabs.org" <openbmc@lists.ozlabs.org>,
        "joel@jms.id.au" <joel@jms.id.au>
Date:   Mon, 19 Jul 2021 10:57:19 -0500
In-Reply-To: <0ed7c393b7aa4252a48cd2397ac251eb@AcuMS.aculab.com>
References: <20210716133915.14697-1-eajames@linux.ibm.com>
         <0ed7c393b7aa4252a48cd2397ac251eb@AcuMS.aculab.com>
Organization: IBM
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Sat, 2021-07-17 at 13:46 +0000, David Laight wrote:
> From: Eddie James
> > Sent: 16 July 2021 14:39
> > 
> > The security restrictions on the FSI-attached SPI controllers have
> > been applied universally to all controllers, so the controller can
> > no
> > longer transfer more than 8 bytes for one transfer. Refactor the
> > driver
> > to remove the looping and support for larger transfers, and remove
> > the
> > "restricted" compatible string, as all the controllers are now
> > considered restricted.
> 
> Aren't there significant performance (and device wear?) penalties
> for doing short SPI eeprom writes?

Probably so. However there is no choice in the matter, as the SPI
controller can't process larger reads/writes.
(I should note that the controller/driver CAN process up to 40 byte
writes. However, the driver must report the minimum of read and write
sizes in the max_transfer_size callback, so no client driver should
request larger than the max read size - 8 bytes).

Thanks,
Eddie

> 
> IIRC (and I'm not getting my serial busses confused) a write request
> can request an aligned transfer of up to (typically) 32 bytes.
> At which point you need to wait for the status to indicate
> 'complete'.
> 
> So restricting writes to 8 bytes increases block write times
> by a factor of 4.
> 
> Increasing the numbers of writes by a factor or 4 may also have
> an effect on device wear - but that is more likely only affected
> by erase cycles.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
> MK1 1PT, UK
> Registration No: 1397386 (Wales)
>