Re: [PATCH v3 1/3] lib: bitmap: introduce bitmap_print_to_buf

From: Andy Shevchenko
Date: Thu Jun 03 2021 - 09:00:41 EST


On Thu, Jun 03, 2021 at 01:39:19PM +0100, Jonathan Cameron wrote:
> On Thu, 3 Jun 2021 14:11:16 +0300
> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> > On Thu, Jun 03, 2021 at 06:33:25PM +0800, tiantao (H) wrote:
> > > 在 2021/6/3 17:50, Andy Shevchenko 写道:
> > > > On Thu, Jun 03, 2021 at 05:22:40PM +0800, Tian Tao wrote:
> > > > > New API bitmap_print_to_buf() with bin_attribute to avoid maskp
> > > > > exceeding PAGE_SIZE. bitmap_print_to_pagebuf() is a special case
> > > > > of bitmap_print_to_buf(), so in bitmap_print_to_pagebuf() call
> > > > > bitmap_print_to_buf().
> >
> > ...
> >
> > > > > + * @count: the maximum number of bytes to print
> >
> > > > > + size = memory_read_from_buffer(buf, count, &off, data, strlen(data) + 1);
> > > > Are you sure you have put parameters in the correct order?
> > >
> > > yes, I already test it.
> > >
> > > ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
> > >                                 const void *from, size_t available)
> >
> > Have you read the meaning of count and available?
> > Please, double check that they are filled with correct values.
>
> Ok, I don't get this one either so can you give us more of a hint?

There is no hint, as you noticed the documentation of the function is a bit
confusing.

> /**
> * memory_read_from_buffer - copy data from the buffer
> * @to: the kernel space buffer to read to
> * @count: the maximum number of bytes to read
> * @ppos: the current position in the buffer
> * @from: the buffer to read from
> * @available: the size of the buffer
> *
> * The memory_read_from_buffer() function reads up to @count bytes from the
> * buffer @from at offset @ppos into the kernel space address starting at @to.
> *
> * On success, the number of bytes read is returned and the offset @ppos is
> * advanced by this number, or negative value is returned on error.
> **/
>
> These docs do end up rather confusing by using the term buffer for multiple things
> but taking what is passed in.
>
> Count is the maximum in the sense of how many bytes we are requesting are read
> which indeed should be count here as that reflects what userspace asked for.
>
> Avail is the size of the buffer we are reading from. Now that's slightly
> ambiguous in the docs in the sense of 'buffer' could mean the to buffer or
> the from buffer. However, I'd assume count is definitely <= size of the space
> after address to in the to buffer, so I would assume that means available
> is the size of the from buffer. Here that is strlen() + 1, so looks fine.
>
> This interpretation also lines up with the implementation.
>
> So what are we missing?

Thanks for double checking and explaining.

> > > > I guess you have to provide the test case(s).

Just test cases is what we are missing. Then we can play around with different
input to see if it's all correct.

--
With Best Regards,
Andy Shevchenko