Re: [PATCH v2 3/3] arm64: dts: qcom: apq8016: Add Schneider HMIBSC board DTS

[Stephan Gerhold <stephan@gerhold.net>]

On Thu, Mar 14, 2024 at 03:02:31PM +0530, Sumit Garg wrote:
> On Thu, 14 Mar 2024 at 14:48, Konrad Dybcio <konrad.dybcio@linaro.org> wrote:
> > On 3/14/24 10:04, Sumit Garg wrote:
> > > On Wed, 13 Mar 2024 at 18:34, Konrad Dybcio <konrad.dybcio@linaro.org> wrote:
> > >> On 3/13/24 13:30, Sumit Garg wrote:
> > >>> Add Schneider Electric HMIBSC board DTS. The HMIBSC board is an IIoT Edge
> > >>> Box Core board based on the Qualcomm APQ8016E SoC.
> > >>>
> > >>> Support for Schneider Electric HMIBSC. Features:
> > >>> - Qualcomm Snapdragon 410C SoC - APQ8016 (4xCortex A53, Adreno 306)
> > >>> - 1GiB RAM
> > >>> - 8GiB eMMC, SD slot
> > >>> - WiFi and Bluetooth
> > >>> - 2x Host, 1x Device USB port
> > >>> - HDMI
> > >>> - Discrete TPM2 chip over SPI
> > >>> - USB ethernet adaptors (soldered)
> > >>>
> > >>> Co-developed-by: Jagdish Gediya <jagdish.gediya@linaro.org>
> > >>> Signed-off-by: Jagdish Gediya <jagdish.gediya@linaro.org>
> > >>> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > >>> ---
> > >>
> > >> [...]
> > >>
> > >>> +     memory@80000000 {
> > >>> +             reg = <0 0x80000000 0 0x40000000>;
> > >>> +     };
> > >>
> > >> I'm not sure the entirety of DRAM is accessible..
> > >>
> > >> This override should be unnecessary, as bootloaders generally update
> > >> the size field anyway.
> > >
> > > On this board, U-Boot is used as the first stage bootloader (replacing
> > > Little Kernel (LK), thanks to Stephan's work). And U-Boot consumes
> > > memory range from DT as Linux does but doesn't require any memory to
> > > be reserved for U-Boot itself. So apart from reserved memory nodes
> > > explicitly described in DT all the other DRAM regions are accessible.
> >
> > Still, u-boot has code to fetch the size dynamically, no?
> >
> 
> No U-Boot being the first stage bootloader fetches size from DT which
> is bundled into U-Boot binary.
> 

Back when I added support for using U-Boot as first stage bootloader on
DB410c the way it worked is that U-Boot used a fixed amount of DRAM
(originally 968 MiB, later 1 GiB since I fixed this in commit
1d667227ea51 ("board: dragonboard410c: Fix PHYS_SDRAM_1_SIZE") [1]).
When booting Linux, the Linux DT was dynamically patched with the right
amount of DRAM (obtained from SMEM). So if you had e.g. a Geniatech DB4
board with 2 GiB DRAM, U-Boot was only using 1 GiB of DRAM, but Linux
later got the full 2 GiB patched into its DTB.

I didn't have much time for testing U-Boot myself lately but a quick
look at the recent changes suggest that Caleb accidentally removed that
functionality in the recent cleanup. Specifically, the SMEM-based DRAM
size detection was removed in commit 14868845db54 ("board:
dragonboard410c: import board code from mach-snapdragon" [2]), the
msm_fixup_memory() function does not seem to exist anymore now. :')

Also, the DRAM size is now always taken from the DT (which is probably
better than the previous hardcoded size in the U-Boot board code).

I think we should bring the dynamic DRAM size detection back, because
there are quite some boards available with varying DRAM size. Restoring
msm_fixup_memory() would likely be easiest, I guess the ideal solution
would be to parse SMEM in U-Boot's dram_init() function so even U-Boot
has the correct amount of DRAM to work with.

Thanks,
Stephan

[1]: https://source.denx.de/u-boot/u-boot/-/commit/1d667227ea512537b8453abeb49abbf19a1a18e8
[2]: https://source.denx.de/u-boot/u-boot/-/commit/14868845db54b4f64701977385dc9a6e951e4139