Re: [PATCH v4 3/4] mtd: spi-nor: otp: return -EROFS if region is read-only

[Pratyush Yadav]

On 26/05/21 12:41PM, Michael Walle wrote:
> Am 2021-05-25 21:33, schrieb Pratyush Yadav:
> > On 21/05/21 09:40PM, Michael Walle wrote:
> > > SPI NOR flashes will just ignore program commands if the OTP region is
> > > locked. Thus, a user might not notice that the intended write didn't
> > > end
> > > up in the flash. Return -EROFS to the user in this case. From what I
> > > can
> > > tell, chips/cfi_cmdset_0001.c also return this error code.
> > > 
> > > One could optimize spi_nor_mtd_otp_range_is_locked() to read the
> > > status
> > > register only once and not for every OTP region, but for that we would
> > > need some more invasive changes. Given that this is
> > > one-time-programmable memory and the normal access mode is reading, we
> > > just live with the small overhead.
> > 
> > Ok.
> > 
> > > 
> > > Fixes: 069089acf88b ("mtd: spi-nor: add OTP support")
> > > Signed-off-by: Michael Walle <michael@xxxxxxxx>
> > > ---
> > >  drivers/mtd/spi-nor/otp.c | 35 +++++++++++++++++++++++++++++++++++
> > >  1 file changed, 35 insertions(+)
> > > 
> > > diff --git a/drivers/mtd/spi-nor/otp.c b/drivers/mtd/spi-nor/otp.c
> > > index 3898ed67ba1c..b87f96593c13 100644
> > > --- a/drivers/mtd/spi-nor/otp.c
> > > +++ b/drivers/mtd/spi-nor/otp.c
> > > @@ -249,6 +249,31 @@ static int spi_nor_mtd_otp_info(struct mtd_info
> > > *mtd, size_t len,
> > >  	return ret;
> > >  }
> > > 
> > > +static int spi_nor_mtd_otp_range_is_locked(struct spi_nor *nor,
> > > loff_t ofs,
> > > +					   size_t len)
> > > +{
> > > +	const struct spi_nor_otp_ops *ops = nor->params->otp.ops;
> > > +	unsigned int region;
> > > +	int locked;
> > > +
> > > +	if (!len)
> > > +		return 0;
> > 
> > I was inclined to say that the loop conditional below would take care of
> > this but it can cause an underflow when ofs and len are both 0.
> 
> Correct. I didn't want to put an extra check to the caller, because
> as you noticed, it is checked by the loop there later.
> 
> > > +
> > > +	/*
> > > +	 * If any of the affected OTP regions are locked the entire range is
> > > +	 * considered locked.
> > > +	 */
> > > +	for (region = spi_nor_otp_offset_to_region(nor, ofs);
> > > +	     region <= spi_nor_otp_offset_to_region(nor, ofs + len - 1);
> > > +	     region++) {
> > > +		locked = ops->is_locked(nor, region);
> > > +		if (locked)
> > > +			return locked;
> > > +	}
> > 
> > Ok.
> 
> Btw I didn't know if I should put a comment here that this if () handles
> both locked state and errors. But it seems you've already found out by
> looking at the caller ;) I'm not sure if this is obvious, though.

I didn't catch this on the first read. I only figured it out when I 
looked at the return check below. So it is certainly not obvious.

> 
> > > +
> > > +	return 0;
> > > +}
> > > +
> > >  static int spi_nor_mtd_otp_read_write(struct mtd_info *mtd, loff_t
> > > ofs,
> > >  				      size_t total_len, size_t *retlen,
> > >  				      const u8 *buf, bool is_write)
> > > @@ -271,6 +296,16 @@ static int spi_nor_mtd_otp_read_write(struct
> > > mtd_info *mtd, loff_t ofs,
> > >  	/* don't access beyond the end */
> > >  	total_len = min_t(size_t, total_len, spi_nor_otp_size(nor) - ofs);
> > > 
> > > +	if (is_write) {
> > > +		ret = spi_nor_mtd_otp_range_is_locked(nor, ofs, total_len);
> > > +		if (ret < 0) {
> > > +			goto out;
> > > +		} else if (ret) {
> > > +			ret = -EROFS;
> > 
> > I wonder if we should have a dev_info() or dev_err() here. I think this
> > warrants a dev_dbg() at least.
> 
> Are you sure? Reporting something to the user via an error code is
> enough IMHO. I wouldn't want my syslog to be cluttered with messages
> I already know. I mean the program tell me "hey, you aren't allowed
> to write there". Why would the kernel still need to tell me that again?
> Without any connection to the caller, I don't get much out of the kernel
> message by looking at it alone, just that someone tried to write there.
> 
> So definetly no dev_info() or dev_err(). But IMHO no dev_dbg() either.
> Tudor, Vingesh, any opinions?

Either is fine by me.

> 
> 
> > > +			goto out;
> > > +		}
> > 
> > So it returns -errno when the check for is_locked() fails and 1 or 0
> > when it is locked or not. Ok.
> > 
> > It would be nice if you add a dev_dbg or dev_err() or dev_info() above.
> > Nonetheless,
> > 
> > Reviewed-by: Pratyush Yadav <p.yadav@xxxxxx>
> 
> Thanks for reviewing!
> 
> -michael

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.