linux-kernel - Re: [PATCH 08/10] spi: aspeed: Calibrate read timings

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Date:   Sun, 27 Feb 2022 22:27:52 +0100
From:   Cédric Le Goater <clg@...d.org>
To:     Pratyush Yadav <p.yadav@...com>
CC:     <linux-spi@...r.kernel.org>, <linux-mtd@...ts.infradead.org>,
        Mark Brown <broonie@...nel.org>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        <linux-aspeed@...ts.ozlabs.org>, Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Chin-Ting Kuo <chin-ting_kuo@...eedtech.com>,
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/10] spi: aspeed: Calibrate read timings

On 2/25/22 10:18, Pratyush Yadav wrote:
> On 14/02/22 10:42AM, Cédric Le Goater wrote:
>> To accommodate the different response time of SPI transfers on different
>> boards and different SPI NOR devices, the Aspeed controllers provide a
>> set of Read Timing Compensation registers to tune the timing delays
>> depending on the frequency being used. The AST2600 SoC has one of
>> these registers per device. On the AST2500 and AST2400 SoCs, the
>> timing register is shared by all devices which is a bit problematic to
>> get good results other than for one device.
>>
>> The algorithm first reads a golden buffer at low speed and then performs
>> reads with different clocks and delay cycle settings to find a breaking
>> point. This selects a default good frequency for the CEx control register.
>> The current settings are bit optimistic as we pick the first delay giving
>> good results. A safer approach would be to determine an interval and
>> choose the middle value.
>>
>> Due to the lack of API, calibration is performed when the direct mapping
>> for reads is created.
> 
> The dirmap_create mapping says nothing about _when_ it should be called.
> So there is no guarantee that it will only be called after the flash is
> fully initialized. 

spi_nor_create_read_dirmap() is called after spi_nor_scan() in spi_nor_probe().
Since a spi_mem_dirmap_info descriptor is created using the nor fields :

	struct spi_mem_dirmap_info info = {
		.op_tmpl = SPI_MEM_OP(SPI_MEM_OP_CMD(nor->read_opcode, 0),
				      SPI_MEM_OP_ADDR(nor->addr_width, 0, 0),
				      SPI_MEM_OP_DUMMY(nor->read_dummy, 0),
				      SPI_MEM_OP_DATA_IN(0, NULL, 0)),
		.offset = 0,
		.length = nor->params->size,
	};
	struct spi_mem_op *op = &info.op_tmpl;

the spi-mem framework makes the assumption that the nor object is initialized.

> I suggest you either make this a requirement of the API,

how ?

Thanks,

C.


> or create a new API that guarantees it will only be called after
> the flash is initialized, like [0].
> 
> [0] https://patchwork.ozlabs.org/project/linux-mtd/patch/20210311191216.7363-2-p.yadav@ti.com/
> 
>>
>> Cc: Pratyush Yadav <p.yadav@...com>
>> Signed-off-by: Cédric Le Goater <clg@...d.org>
>> ---
>