Re: [PATCH v4 3/4] remoteproc: imx_dsp_rproc: Add remoteproc driver for DSP on i.MX

[Shengjiu Wang <shengjiu.wang@gmail.com>]

Hi Mathieu

On Fri, Sep 17, 2021 at 11:22 PM Mathieu Poirier
<mathieu.poirier@linaro.org> wrote:
>
> On Fri, Sep 17, 2021 at 05:44:44PM +0800, Shengjiu Wang wrote:
> > On Fri, Sep 17, 2021 at 1:20 PM Shengjiu Wang <shengjiu.wang@gmail.com> wrote:
> > >
> > > On Fri, Sep 17, 2021 at 1:00 AM Mathieu Poirier
> > > <mathieu.poirier@linaro.org> wrote:
> > > >
> > > > [...]
> > > >
> > > > > > > +
> > > > > > > +/**
> > > > > > > + * imx_dsp_rproc_elf_load_segments() - load firmware segments to memory
> > > > > > > + * @rproc: remote processor which will be booted using these fw segments
> > > > > > > + * @fw: the ELF firmware image
> > > > > > > + *
> > > > > > > + * This function specially checks if memsz is zero or not, otherwise it
> > > > > > > + * is mostly same as rproc_elf_load_segments().
> > > > > > > + */
> > > > > > > +static int imx_dsp_rproc_elf_load_segments(struct rproc *rproc,
> > > > > > > +                                        const struct firmware *fw)
> > > > > > > +{
> > > > > > > +     struct device *dev = &rproc->dev;
> > > > > > > +     u8 class = fw_elf_get_class(fw);
> > > > > > > +     u32 elf_phdr_get_size = elf_size_of_phdr(class);
> > > > > > > +     const u8 *elf_data = fw->data;
> > > > > > > +     const void *ehdr, *phdr;
> > > > > > > +     int i, ret = 0;
> > > > > > > +     u16 phnum;
> > > > > > > +
> > > > > > > +     ehdr = elf_data;
> > > > > > > +     phnum = elf_hdr_get_e_phnum(class, ehdr);
> > > > > > > +     phdr = elf_data + elf_hdr_get_e_phoff(class, ehdr);
> > > > > > > +
> > > > > > > +     /* go through the available ELF segments */
> > > > > > > +     for (i = 0; i < phnum; i++, phdr += elf_phdr_get_size) {
> > > > > > > +             u64 da = elf_phdr_get_p_paddr(class, phdr);
> > > > > > > +             u64 memsz = elf_phdr_get_p_memsz(class, phdr);
> > > > > > > +             u64 filesz = elf_phdr_get_p_filesz(class, phdr);
> > > > > > > +             u64 offset = elf_phdr_get_p_offset(class, phdr);
> > > > > > > +             u32 type = elf_phdr_get_p_type(class, phdr);
> > > > > > > +             void *ptr;
> > > > > > > +             bool is_iomem;
> > > > > > > +
> > > > > > > +             if (type != PT_LOAD || !memsz)
> > > > > >
> > > > > > You did a really good job with adding comments but this part is undocumented...
> > > > > > If I read this correctly you need to check for !memsz because some part of
> > > > > > the program segment may have a header but its memsz is zero, in which case it can
> > > > > > be safely skipped.  So why is that segment in the image to start with, and why
> > > > > > is it marked PT_LOAD if it is not needed?  This is very puzzling...
> > > > >
> > > > > Actually I have added comments in the header of this function.
> > > >
> > > > Indeed there is a mention of memsz in the function's header but it doesn't
> > > > mention _why_ this is needed, and that is what I'm looking for.
> > > >
> > > > >
> > > > > memsz= 0 with PT_LOAD issue, I have asked the toolchain's vendor,
> > > > > they said that this case is allowed by elf spec...
> > > > >
> > > > > And in the "pru_rproc.c" and "mtk_scp.c", seems they met same problem
> > > > > they also check the filesz in their internal xxx_elf_load_segments() function.
> > > >
> > > > In both cases they are skipping PT_LOAD sections where "filesz" is '0', which
> > > > makes sense because we don't know how many bytes to copy.  But here you are
> > > > skipping over a PT_LOAD section with a potentially valid filesz, and that is the
> > > > part I don't understand.
> > >
> > > Ok, I can use filesz instead. For my case, filesz = memsz = 0,
> > > it is the same result I want.
>
> If that is the case then rproc_elf_load_segments() should work, i.e it won't
> copy anything.  If rproc_elf_load_segments() doesn't work for you then there are
> corner cases you haven't told me about.
>
> > >
> > > The reason why I use "memsz '' is because there is  "if (filesz > memsz) "
> > > check after this,  if memsz is zero, then "filesz" should be zero too, other
> > > values are not allowed.
> >
> > But I still think checking "!memsz" is better than filesz,  because
> > memsz > filesz is allowed (filesz = 0),  the code below can be executed.
> > filesz > memsz is not allowed.
> >
> > What do you think?
>
> I don't see a need to add a custom implementation for things that _may_ happen.
> If using the default rproc_elf_load_segments() works than go with that.  We can deal
> with problems if/when there is a need for it.
>

The default rproc_elf_load_segments() with filesz = memsz = 0, then the
rproc_da_to_va() return ptr=NULL, then rproc_elf_load_segments() will return
with error.  So this is the reason to add a custom implementation.

best regards
wang shengjiu