Re: [PATCH v1 07/17] misc: eeprom: at25: Change nvmem reg_read/write return type

[Joy Chakraborty <joychakr@xxxxxxxxxx>]

On Thu, Jun 6, 2024 at 2:11 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> On Wed, Jun 05, 2024 at 05:59:51PM +0000, Joy Chakraborty wrote:
> > @@ -195,10 +195,11 @@ static struct attribute *sernum_attrs[] = {
> >  };
> >  ATTRIBUTE_GROUPS(sernum);
> >
> > -static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
> > +static ssize_t at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
> >  {
> >       struct at25_data *at25 = priv;
> >       size_t maxsz = spi_max_transfer_size(at25->spi);
> > +     size_t bytes_written = count;
> >       const char *buf = val;
> >       int                     status = 0;
> >       unsigned                buf_size;
> > @@ -313,7 +314,7 @@ static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
> >       mutex_unlock(&at25->lock);
> >
> >       kfree(bounce);
> > -     return status;
> > +     return status < 0 ? status : bytes_written;
> >  }
>
> So the original bug was that rmem_read() is returning positive values
> on success instead of zero[1].  That started a discussion about partial
> reads which resulted in changing the API to support partial reads[2].
> That patchset broke the build.  This patchset is trying to fix the
> build breakage.
>
> [1] https://lore.kernel.org/all/20240206042408.224138-1-joychakr@xxxxxxxxxx/
> [2] https://lore.kernel.org/all/20240510082929.3792559-2-joychakr@xxxxxxxxxx/
>
> The bug in rmem_read() is still not fixed.  That needs to be fixed as
> a stand alone patch.  We can discuss re-writing the API separately.
>

True, fixing the return type would fix that as well is what I thought
but maybe yes we need to fix that separately as well.

> These functions are used internally and exported to the user through
> sysfs via bin_attr_nvmem_read/write().  For internal users partial reads
> should be treated as failure.  What are we supposed to do with a partial
> read?  I don't think anyone has asked for partial reads to be supported
> from sysfs either except Greg was wondering about it while reading the
> code.
>
> Currently, a lot of drivers return -EINVAL for partial read/writes but
> some return success.  It is a bit messy.  But this patchset doesn't
> really improve anything.  In at24_read() we check if it's going to be a
> partial read and return -EINVAL.  Below we report a partial read as a
> full read.  It's just a more complicated way of doing exactly what we
> were doing before.

Currently what drivers return is up to their interpretation of int
return type, there are a few drivers which also return the number of
bytes written/read already like
drivers/misc/mchp_pci1xxxx/mchp_pci1xxxx_otpe2p.c .
The objective of the patch was to handle partial reads and errors at
the nvmem core and instead of leaving it up to each nvmem provider by
providing a better return value to nvmem providers.

Regarding drivers/misc/eeprom/at25.c which you pointed below, that is
a problem in my code change. I missed that count was modified later on
and should initialize bytes_written to the new value of count, will
fix that when I come up with the new patch.

I agree that it does not improve anything for a lot of nvmem providers
for example the ones which call into other reg_map_read/write apis
which do not return the number of bytes read/written but it does help
us do better error handling at the nvmem core layer for nvmem
providers who can return the valid number of bytes read/written.

Please let me know if you have any other suggestions on how to handle
this better.

Thanks
Joy

>
> drivers/misc/eeprom/at25.c
>    198  static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
>    199  {
>    200          struct at25_data *at25 = priv;
>    201          size_t maxsz = spi_max_transfer_size(at25->spi);
> New:            size_t bytes_written = count;
>                        ^^^^^^^^^^^^^^^^^^^^^
> This is not the number of bytes written.
>
>    202          const char *buf = val;
>    203          int                     status = 0;
>    204          unsigned                buf_size;
>    205          u8                      *bounce;
>    206
>    207          if (unlikely(off >= at25->chip.byte_len))
>    208                  return -EFBIG;
>    209          if ((off + count) > at25->chip.byte_len)
>    210                  count = at25->chip.byte_len - off;
>                         ^^^^^
> This is.
>
>    211          if (unlikely(!count))
>    212                  return -EINVAL;
>    213
>    214          /* Temp buffer starts with command and address */
>    215          buf_size = at25->chip.page_size;
>    216          if (buf_size > io_limit)
>    217                  buf_size = io_limit;
>    218          bounce = kmalloc(buf_size + at25->addrlen + 1, GFP_KERNEL);
>    219          if (!bounce)
>    220                  return -ENOMEM;
>    221
>
> regards,
> dan carpenter