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Date: Thu, 6 Jun 2024 13:10:58 +0300
From: Dan Carpenter <dan.carpenter@linaro.org>
To: Joy Chakraborty <joychakr@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
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	Sakari Ailus <sakari.ailus@linux.intel.com>,
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Subject: Re: [PATCH v1 07/17] misc: eeprom: at25: Change nvmem reg_read/write
 return type
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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.

> 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.

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

regards,
dan carpenter