linux-kernel - Re: [RFC PATCH 3/6] mtd: spi-nor: core: run calibration when initialization is done

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Message-ID: <b3bfa5a6-caac-94ed-6741-04db9c2a9ee0@kaod.org>
Date:   Wed, 18 May 2022 10:51:58 +0200
From:   Cédric Le Goater <clg@...d.org>
To:     Pratyush Yadav <p.yadav@...com>,
        Miquel Raynal <miquel.raynal@...tlin.com>
CC:     Nishanth Menon <nm@...com>, Tero Kristo <kristo@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Michael Walle <michael@...le.cc>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Mark Brown <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>,
        Joel Stanley <joel@....id.au>
Subject: Re: [RFC PATCH 3/6] mtd: spi-nor: core: run calibration when
 initialization is done

Hello,

On 5/18/22 09:56, Pratyush Yadav wrote:
> On 18/05/22 09:19AM, Miquel Raynal wrote:
>> Hi Pratyush,
>>
>> p.yadav@...com wrote on Wed, 18 May 2022 11:37:05 +0530:
>>
>>> +Cedric
>>>
>>> On 17/05/22 04:02PM, Miquel Raynal wrote:
>>>> Hi Pratyush,
>>>>
>>>> p.yadav@...com wrote on Fri, 12 Mar 2021 00:42:13 +0530:
>>>>    
>>>>> Once the flash is initialized tell the controller it can run
>>>>> calibration procedures if needed. This can be useful when calibration is
>>>>> needed to run at higher clock speeds.
>>>>>
>>>>> Signed-off-by: Pratyush Yadav <p.yadav@...com>
>>>>> ---
>>>>>   drivers/mtd/spi-nor/core.c | 12 ++++++++++--
>>>>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
>>>>> index 88888df009f0..e0cbcaf1be89 100644
>>>>> --- a/drivers/mtd/spi-nor/core.c
>>>>> +++ b/drivers/mtd/spi-nor/core.c
>>>>> @@ -3650,6 +3650,7 @@ static int spi_nor_probe(struct spi_mem *spimem)
>>>>>   	 * checking what's really supported using spi_mem_supports_op().
>>>>>   	 */
>>>>>   	const struct spi_nor_hwcaps hwcaps = { .mask = SNOR_HWCAPS_ALL };
>>>>> +	struct spi_mem_op op;
>>>>>   	char *flash_name;
>>>>>   	int ret;
>>>>>   
>>>>> @@ -3709,8 +3710,15 @@ static int spi_nor_probe(struct spi_mem *spimem)
>>>>>   	if (ret)
>>>>>   		return ret;
>>>>>   
>>>>> -	return mtd_device_register(&nor->mtd, data ? data->parts : NULL,
>>>>> -				   data ? data->nr_parts : 0);
>>>>> +	ret = mtd_device_register(&nor->mtd, data ? data->parts : NULL,
>>>>> +				  data ? data->nr_parts : 0);
>>>>> +	if (ret)
>>>>> +		return ret;
>>>>> +
>>>>> +	op = spi_nor_spimem_get_read_op(nor);
>>>>
>>>> Isn't this too specific? I really don't know much about spi-nors, but I
>>>> find odd to have this op being created here, why not moving this into
>>>> the _do_calibration() helper?
>>>
>>> Maybe the naming confused you but this is a function in the SPI NOR
>>> core, not in SPI MEM. SPI NOR supports both SPI MEM based controllers
>>> and "legacy" controllers, so the convention is to add the "spimem"
>>> prefix before SPI MEM specific functions. So I don't get the comment
>>> about it being too specific. It is too specific to what?
>>
>> Mmh right, it's fine then.
>>
>>>
>>> And how can spi_mem_do_calibration() know what op the flash uses to read
>>> data? SPI NOR or SPI NAND would know it, but not SPI MEM. That is why we
>>> pass in that information to spi_mem_do_calibration().
>>
>> But here the op is "spi-nor wide", I would have expected a
>> per-device op. But that is not a big deal, that is something that can
>> also be updated later if needed I guess.
> 
> It is per-device. The op is generated using nor->read_opcode,
> nor->addr_width, nor->read_dummy, etc. So if you have 2 NOR flashes on
> your system with different opcodes, it would work for both.
> 
>>
>> One last question, is there something that mtd_device_register() does
>> that is really needed for the calibration to work? Otherwise I would
>> rather prefer to have that calibration happening before the user gets
>> access to the device.

Which would mean calling it right after :

	ret = spi_nor_create_read_dirmap(nor);
	if (ret)
		return ret;

	ret = spi_nor_create_write_dirmap(nor);
	if (ret)
		return ret;

> The calibration works by reading a known pattern that is already written
> to the flash again and again and seeing what delays work and what don't.
> For that to happen, the controller driver needs to know where the
> pattern is stored. 

Why don't you simply choose some random place, first 16KB for instance,
and check that the data is random enough ? If not, declare calibration
not possible and choose a default safe setting which is anyhow a
requirement for calibration. Retry at reboot as data might have changed.

> This series does that by looking at the MTD
> partitions. For that to happen, we need to create those partitions
> first, which happens after mtd_device_register().

hmm, that might work for some boards. This is not at all the case for
the BMC boards. Vendors can put any kind of flash model and/or layout
and the driver needs to be more generic.

> But I am planning to use device tree to get that information now so this
> should no longer be needed and we can do calibration before registering
> the device with MTD.

Perfect, we can move the calibration hook in spi_nor_create_read_dirmap()
then, or in devm_spi_mem_dirmap_create(), which would make more sense IMHO.

Thanks,

C.