linux-kernel - Re: [PATCH] spi: cadence-quadspi: Add upper limit safety check to baudrate divisor

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Date:   Thu, 24 Nov 2022 12:16:10 +0530
From:   Dhruva Gole <d-gole@...com>
To:     Nathan Barrett-Morrison <nathan.morrison@...esys.com>
CC:     <greg.malysa@...esys.com>, Mark Brown <broonie@...nel.org>,
        "open list:SPI SUBSYSTEM" <linux-spi@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] spi: cadence-quadspi: Add upper limit safety check to
 baudrate divisor

Hi Nathan,
Thanks for your contribution.
However, there are a few issues that I would like you to address.

On 24/11/22 02:47, Nathan Barrett-Morrison wrote:
> While bringing up the cadence-quadspi driver on a customer board,
> I discovered that the baud divisor calculation can exceed the
> peripheral's maximum in some circumstances.  This will prevent it.
What is the peripheral's maximum? Is the peripheral a flash?
Please define what you mean by "some circumstances".

> 
> Signed-off-by: Nathan Barrett-Morrison <nathan.morrison@...esys.com>
> ---
>   drivers/spi/spi-cadence-quadspi.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/spi/spi-cadence-quadspi.c b/drivers/spi/spi-cadence-quadspi.c
> index 447230547945..250575fb7b0e 100644
> --- a/drivers/spi/spi-cadence-quadspi.c
> +++ b/drivers/spi/spi-cadence-quadspi.c
> @@ -1119,6 +1119,10 @@ static void cqspi_config_baudrate_div(struct cqspi_st *cqspi)
>   	/* Recalculate the baudrate divisor based on QSPI specification. */
>   	div = DIV_ROUND_UP(ref_clk_hz, 2 * cqspi->sclk) - 1;
>   
> +	/* Maximum baud divisor */
> +	if (div > CQSPI_REG_CONFIG_BAUD_MASK)

I don't think comparing "greater than" with a MASK is atall a good idea.

> +		div = CQSPI_REG_CONFIG_BAUD_MASK;
I would not encourage this either.


> +
>   	reg = readl(reg_base + CQSPI_REG_CONFIG);
>   	reg &= ~(CQSPI_REG_CONFIG_BAUD_MASK << CQSPI_REG_CONFIG_BAUD_LSB);
>   	reg |= (div & CQSPI_REG_CONFIG_BAUD_MASK) << CQSPI_REG_CONFIG_BAUD_LSB;

Either come up with a better MACRO, or if what I understand
is correct, the peripheral's max value will depend, well
on the _peripheral_ in which case it is that "peripheral" driver's
responsibility to properly tell the controller what to do.


Again, I don't fully understand your situation is as in
what is the peripheral you are using. So please elaborate on that.

Importantly, I would suggest that you _NEVER_ compare ANY value to a
MASK Macro. MASK Macros are meant to MASK bits.



-- 
Thanks and Regards,
Dhruva Gole