Re: [PATCH v2 1/2] mtd: spi-nor: macronix: add support for Macronix octaflash

[Pratyush Yadav <p.yadav@xxxxxx>]

On 05/02/21 10:47AM, Miquel Raynal wrote:
> Hello,
> 
> zhengxunli <zhengxunli@xxxxxxxxxxx> wrote on Fri,  5 Feb 2021 17:36:47
> +0800:
> 
> > The ocatflash is an xSPI compliant octal DTR flash. Add support
> > for using it in octal DTR mode.
> > 
> > Enable Octal DTR mode with 20 dummy cycles to allow running at the
> > maximum supported frequency of 200Mhz.
> > 
> > Try to verify the flash ID to check whether the flash memory in octal
> > DTR mode is correct. When reading ID in OCTAL DTR mode, ID will appear
> > in a repeated manner. ex: ID[0] = 0xc2, ID[1] = 0xc2, ID[2] = 0x94,
> > ID[3] = 0x94... Rearrange the order so that the ID can pass.
> > 
> > Signed-off-by: zhengxunli <zhengxunli@xxxxxxxxxxx>
> > ---
> >  drivers/mtd/spi-nor/macronix.c | 121 +++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 121 insertions(+)
> > 
> > diff --git a/drivers/mtd/spi-nor/macronix.c b/drivers/mtd/spi-nor/macronix.c
> > index 9203aba..7498978 100644
> > --- a/drivers/mtd/spi-nor/macronix.c
> > +++ b/drivers/mtd/spi-nor/macronix.c
> > @@ -8,6 +8,16 @@
> >  
> >  #include "core.h"
> >  
> > +#define SPINOR_OP_RD_CR2		0x71		/* Read configuration register 2 */
> > +#define SPINOR_OP_WR_CR2		0x72		/* Write configuration register 2 */
> > +#define SPINOR_OP_MXIC_DTR_RD		0xee		/* Fast Read opcode in DTR mode */
> > +#define SPINOR_REG_MXIC_CR2_MODE	0x00000000	/* For setting octal DTR mode */
> > +#define SPINOR_REG_MXIC_OPI_DTR_EN	0x2		/* Enable Octal DTR */
> > +#define SPINOR_REG_MXIC_OPI_DTR_DIS	0x1		/* Disable Octal DTR */
> > +#define SPINOR_REG_MXIC_CR2_DC		0x00000300	/* For setting dummy cycles */
> > +#define SPINOR_REG_MXIC_DC_20		0x0		/* Setting dummy cycles to 20 */
> > +#define MXIC_MAX_DC			20		/* Maximum value of dummy cycles */
> > +
> >  static int
> >  mx25l25635_post_bfpt_fixups(struct spi_nor *nor,
> >  			    const struct sfdp_parameter_header *bfpt_header,
> > @@ -33,6 +43,113 @@
> >  	.post_bfpt = mx25l25635_post_bfpt_fixups,
> >  };
> >  
> > +/**
> > + * spi_nor_macronix_octal_dtr_enable() - Enable octal DTR on Macronix flashes.
> > + * @nor:		pointer to a 'struct spi_nor'
> > + * @enable:		whether to enable or disable Octal DTR
> > + *
> > + * This also sets the memory access dummy cycles to 20 to allow the flash to
> > + * run at up to 200MHz.
> > + *
> > + * Return: 0 on success, -errno otherwise.
> > + */
> > +static int spi_nor_macronix_octal_dtr_enable(struct spi_nor *nor, bool enable)
> > +{
> > +	struct spi_mem_op op;
> > +	u8 *buf = nor->bouncebuf, i;
> > +	int ret;
> > +
> > +	if (enable) {
> > +		/* Use 20 dummy cycles for memory array reads. */
> > +		ret = spi_nor_write_enable(nor);
> > +		if (ret)
> > +			return ret;
> > +
> > +		*buf = SPINOR_REG_MXIC_DC_20;
> > +		op = (struct spi_mem_op)
> > +			SPI_MEM_OP(SPI_MEM_OP_CMD(SPINOR_OP_WR_CR2, 1),
> > +				   SPI_MEM_OP_ADDR(4, SPINOR_REG_MXIC_CR2_DC, 1),
> > +				   SPI_MEM_OP_NO_DUMMY,
> > +				   SPI_MEM_OP_DATA_OUT(1, buf, 1));
> > +
> > +		ret = spi_mem_exec_op(nor->spimem, &op);
> > +		if (ret)
> > +			return ret;
> > +
> > +		ret = spi_nor_wait_till_ready(nor);
> > +		if (ret)
> > +			return ret;
> > +
> > +		nor->read_dummy = MXIC_MAX_DC;
> 
> I am still not convinced by this constant value.

I think a constant value is fine. This dummy cycle value reflects how 
many cycles the master clock would go through before the flash starts 
emitting the data. If the master (aka the controller) is running at a 
lower frequency then those cycles go through slower, but the flash still 
waits for them to finish before emitting data. And since the master is 
driving the clock and the flash is just "reading" it, both remain in 
sync.

The dummy cycles need to be set for the worst case scenario [0]. The 
flash usually needs a minimum amount of time before it is ready to emit 
the data. So for example if the master is at 25 MHz, the clock period is 
longer so 8 clock cycles [1] might be long enough to exceed that minimum 
time. But when the master is running at 200 MHz, the clock period is 
smaller so 8 cycles might not give the flash enough time to prepare. So 
we need to to wait at least 20 cycles [1] before emitting data.

This is what my patches do for the Cypress S28 flash. I have tested it 
on both 25 MHz and 166 MHz with 22 dummy cycles. It is not the most 
efficient at 25 MHz since 5 dummy cycles is all that is needed for that 
speed, but its the best we can do right now.

[0] Since SPI NOR has no way of knowing what speed the controller is 
running at, assume the fastest speed the flash can run at.
[1] Hypothetical example. Don't know the actual values for this flash.
 
> The rest looks good to me.

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.