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

[Joy Chakraborty]

On Thu, Jun 6, 2024 at 3:41 PM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> On Thu, Jun 06, 2024 at 03:12:03PM +0530, Joy Chakraborty wrote:
> > > 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 .
>
> Returning non-zero is a bug.  It won't break bin_attr_nvmem_read/write()
> but it will break other places like nvmem_access_with_keepouts(),
> __nvmem_cell_read() and nvmem_cell_prepare_write_buffer() where all
> non-zero returns from nvmem_reg_read() are treated as an error.
>

Yes, I will resend the patch to fix that.

> > 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.
>
> If we're going to support partial writes, then it needs to be done all
> the way.  We need to audit functions like at24_read() and remove the
> -EINVAL lines.
>
>    440          if (off + count > at24->byte_len)
>    441                  return -EINVAL;
>
> It should be:
>
>         if (off + count > at24->byte_len)
>                 count = at24->byte_len - off;
>
> Some drivers handle writing zero bytes as -EINVAL and some return 0.
> Those changes could be done before we change the API.
>

Sure, we can do it in a phased manner like you suggested in another
reply by creating new pointers and slowly moving each driver to the
new pointer and then deprecating the old one.

> You updated nvmem_access_with_keepouts() to handle negative returns but
> not zero returns so it could lead to a forever loop.
>

Yes, that is a possible case. Will rework it.

> regards,
> dan carpenter
>
Thanks
Joy