Re: [RFT PATCH v3 1/9] RISC-V: Move DT mapping outof fixmap

[Atish Patra]

On Sat, Jul 18, 2020 at 2:24 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Sat, Jul 18, 2020 at 3:05 AM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> > On Thu, Jul 16, 2020 at 11:32 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > On Fri, Jul 17, 2020 at 1:41 AM Atish Patra <atish.patra@xxxxxxx> wrote:
> > > > +#define DTB_EARLY_SIZE         SZ_1M
> > > > +static char early_dtb[DTB_EARLY_SIZE] __initdata;
> > >
> > > Hardcoding the size in .bss seems slightly problematic both for
> > > small and for big systems. On machines with very little memory,
> > > this can lead to running out of free pages before .initdata gets freed,
> > > and it increases the size of the uncompressed vmlinux file by quite
> > > a bit.
> > >
> > > On some systems, the 1MB limit may get too small. While most dtbs
> > > would fall into the range between 10KB and 100KB, it can also be
> > > much larger than that, e.g. when there are DT properties that include
> > > blobs like device firmware that gets loaded into hardware by a kernel
> > > driver.
> > >
> > I was not aware that we can do such things. Is there a current example of that ?
>
> I worked on a product in the distant past where the host firmware
> included the ethernet controller firmware as a DT property[1] to get around
> restrictions on redistributing the blob in the linux-firmware package.
>
> For the .dts files we distribute with the kernel, that would not make
> sense, and I don't know of any current machines that do this in their
> system firmware.
>
> > > Is there anything stopping you from parsing the FDT in its original
> > > location without the extra copy before it gets unflattened?
> >
> > That's what the original code was doing. A fixmap entry was added to
> > map the original fdt
> > location to a virtual so that parse_dtb can be operated on a virtual
> > address. But we can't map
> > both FDT & early ioremap within a single PMD region( 2MB ). That's why
> > we removed the DT
> > mapping from the fixmap to .bss section. The other alternate option is
> > to increase the fixmap space to 4MB which seems more fragile.
>
> Could the original location just be part of the regular linear mapping of all
> RAM?

No. Because we don't map the entire RAM until setup_vm_final().
We need to parse DT before setup_vm_final() to get the memblocks and
reserved memory regions.

I'm not too familiar with the early mapping code myself, so it may not
> be possible, but that would be the most logical place where I'd put it.
>
>         Arnd
>
> [1] drivers/net/ethernet/toshiba/spider_net.c



-- 
Regards,
Atish