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SubjectRE: [LINUX PATCH v2 3/3] spi: spi-mem: Add support for Zynq QSPI controller
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Hi Vignesh,

Thanks for the review.

> -----Original Message-----
> From: Vignesh Raghavendra <vigneshr@ti.com>
> Sent: Friday, April 5, 2019 10:14 AM
> To: Naga Sureshkumar Relli <nagasure@xilinx.com>; broonie@kernel.org;
> bbrezillon@kernel.org
> Cc: linux-spi@vger.kernel.org; dwmw2@infradead.org; marek.vasut@gmail.com;
> richard@nod.at; linux-mtd@lists.infradead.org; linux-kernel@vger.kernel.org; Michal Simek
> <michals@xilinx.com>; nagasuresh12@gmail.com
> Subject: Re: [LINUX PATCH v2 3/3] spi: spi-mem: Add support for Zynq QSPI controller
>
>
>
> On 01/04/19 1:29 PM, Naga Sureshkumar Relli wrote:
> > +/**
> > + * zynq_qspi_config_op - Configure QSPI controller for specified transfer
> > + * @xqspi: Pointer to the zynq_qspi structure
> > + * @qspi: Pointer to the spi_device structure
> > + *
> > + * Sets the operational mode of QSPI controller for the next QSPI
> > +transfer and
> > + * sets the requested clock frequency.
> > + *
> > + * Return: 0 on success and -EINVAL on invalid input parameter
> > + *
> > + * Note: If the requested frequency is not an exact match with what
> > +can be
> > + * obtained using the prescalar value, the driver sets the clock
> > +frequency which
> > + * is lower than the requested frequency (maximum lower) for the
> > +transfer. If
> > + * the requested frequency is higher or lower than that is supported
> > +by the QSPI
> > + * controller the driver will set the highest or lowest frequency
> > +supported by
> > + * controller.
> > + */
> > +static int zynq_qspi_config_op(struct zynq_qspi *xqspi, struct
> > +spi_device *spi) {
> > + u32 config_reg, baud_rate_val = 0;
> > +
> > + /*
> > + * Set the clock frequency
> > + * The baud rate divisor is not a direct mapping to the value written
> > + * into the configuration register (config_reg[5:3])
> > + * i.e. 000 - divide by 2
> > + * 001 - divide by 4
> > + * ----------------
> > + * 111 - divide by 256
> > + */
> > + while ((baud_rate_val < ZYNQ_QSPI_BAUD_DIV_MAX) &&
> > + (clk_get_rate(xqspi->refclk) / (2 << baud_rate_val)) >
> > + spi->max_speed_hz)
> > + baud_rate_val++;
> > +
>
> Instead use DIV_ROUND_UP, something like below should work(untested):
>
> unsigned long refclk_rate = clk_get_rate(xqspi->refclk);
> u32 baud_rate_val = DIV_ROUND_UP(refclk_rate, spi->max_speed_hz) - 1;
>
This is not just direct calculation.
i.e. for example
refclk_rate = 200MHz and max_speed_hx = 100MHz.
then DIV_ROUND_UP gives a value of 2.
But writing a value of 2 to config registers means, divide by 8. But we should write divide by 2 (value of zero).
That’s why we implemented the above calculation.

000: divide by 2.
001: divide by 4
010: divide by 8
011: divide by 16
100: divide by 32
101: divide by 64
110: divide by 128
111: divide by 256

Thanks,
Naga Sureshkumar Relli
> if (baud_rate_val > ZYNQ_QSPI_BAUD_DIV_MAX)
> baud_rate_val = ZYNQ_QSPI_BAUD_DIV_MAX;
>
> > + config_reg = zynq_qspi_read(xqspi, ZYNQ_QSPI_CONFIG_OFFSET);
> > +
> > + /* Set the QSPI clock phase and clock polarity */
> > + config_reg &= (~ZYNQ_QSPI_CONFIG_CPHA_MASK) &
> > + (~ZYNQ_QSPI_CONFIG_CPOL_MASK);
> > + if (spi->mode & SPI_CPHA)
> > + config_reg |= ZYNQ_QSPI_CONFIG_CPHA_MASK;
> > + if (spi->mode & SPI_CPOL)
> > + config_reg |= ZYNQ_QSPI_CONFIG_CPOL_MASK;
> > +
> > + config_reg &= ~ZYNQ_QSPI_CONFIG_BDRATE_MASK;
> > + config_reg |= (baud_rate_val << ZYNQ_QSPI_BAUD_DIV_SHIFT);
> > + zynq_qspi_write(xqspi, ZYNQ_QSPI_CONFIG_OFFSET, config_reg);
> > +
> > + return 0;
> > +}
> > +
>
> --
> Regards
> Vignesh
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