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;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Christian Eggers' <ceggers@arri.de>,
        Arnd Bergmann <arnd@arndb.de>,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH] eeprom: at25: set minimum read/write access stride to 1
Thread-Topic: [PATCH] eeprom: at25: set minimum read/write access stride to 1
Thread-Index: AQHWZMG/Rp7Dl/S8M02sb706WA/KmqkcvErQ
Date:   Tue, 28 Jul 2020 09:52:05 +0000
Message-ID: <a65b01608fb34c5c8782b301c2e0cabc@AcuMS.aculab.com>
References: <20200728092959.24600-1-ceggers@arri.de>
In-Reply-To: <20200728092959.24600-1-ceggers@arri.de>
Accept-Language: en-GB, en-US
Content-Language: en-US
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Christian Eggers
> Sent: 28 July 2020 10:30
> 
> SPI eeproms are addressed by byte.

They also support multi-byte writes - possibly with alignment
restrictions.
So forcing 4-byte writes (at aligned addresses) would typically
speed up writes by a factor of 4 over byte writes.

So does this fix a problem?
If so what.

So setting the 'stride' to 4 may be a compromise.
Looking at some code that writes the EPCQ for Altera FPGA
(which I think is just SPI) it does aligned 256 byte writes.
The long writes (and the 4-bit physical interface) are needed
to get the write times down to a sensible value.

	David

> 
> Signed-off-by: Christian Eggers <ceggers@arri.de>
> Cc: stable@vger.kernel.org
> ---
>  drivers/misc/eeprom/at25.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/eeprom/at25.c b/drivers/misc/eeprom/at25.c
> index 0e7c8dc01195..4e57eb145fcc 100644
> --- a/drivers/misc/eeprom/at25.c
> +++ b/drivers/misc/eeprom/at25.c
> @@ -358,7 +358,7 @@ static int at25_probe(struct spi_device *spi)
>  	at25->nvmem_config.reg_read = at25_ee_read;
>  	at25->nvmem_config.reg_write = at25_ee_write;
>  	at25->nvmem_config.priv = at25;
> -	at25->nvmem_config.stride = 4;
> +	at25->nvmem_config.stride = 1;
>  	at25->nvmem_config.word_size = 1;
>  	at25->nvmem_config.size = chip.byte_len;
> 
> --
> Christian Eggers
> Embedded software developer
> 
> Arnold & Richter Cine Technik GmbH & Co. Betriebs KG
> Sitz: Muenchen - Registergericht: Amtsgericht Muenchen - Handelsregisternummer: HRA 57918
> Persoenlich haftender Gesellschafter: Arnold & Richter Cine Technik GmbH
> Sitz: Muenchen - Registergericht: Amtsgericht Muenchen - Handelsregisternummer: HRB 54477
> Geschaeftsfuehrer: Dr. Michael Neuhaeuser; Stephan Schenk; Walter Trauninger; Markus Zeiler

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)