[PATCH] efi_loader: remove EFI_BOUNCE_BUFFER

[Ilias Apalodimas]

Hi Simon

On Thu, 27 Mar 2025 at 15:33, Simon Glass <sjg at chromium.org> wrote:
>
> Hi Ilias,
>
> On Wed, 26 Mar 2025 at 02:37, Ilias Apalodimas
> <ilias.apalodimas at linaro.org> wrote:
> >
> > Hi Heinrich,
> >
> > On Mon, 24 Mar 2025 at 19:50, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> > >
> > > On 17.03.25 14:38, Ilias Apalodimas wrote:
> > >
> > > %s/EFI_BOUNCE_BUFFER/CONFIG_EFI_LOADER_BOUNCE_BUFFER/
> > >
> > > > The EFI subsystem defines its own bounce buffer for devices that
> > > > can't transfer data > 4GB. U-Boot already has a generic BOUNCE_BUFFER
> > > > which can be reused instead of defining another symbol.
> > > > The only limitation for EFI is that we don't know how big the file a user
> > > > chooses to transfer is and as a result we can't depend on allocating the
> > > > memory from the malloc area, which can prove too small.
> > > >
> > > > So allocate an EFI buffer of the correct size and pass it to the DM,
> > > > which already supports bounce buffering via bounce_buffer_start_extalign()
> > >
> > > Looking at
> > >
> > >      if (IS_ENABLED(CONFIG_BOUNCE_BUFFER) && desc->bb) {
> > >
> > > in drivers/block/blk-uclass.c the bounce buffer has to be explicitly
> > > enabled by the device driver. Only the scsi drivers sets bb = true.
> > >
> > > Cf. 81bd22e935dc ("rockchip: block: blk-uclass: add bounce buffer flag
> > > to blk_desc")
> > >
> > > Which device-drivers of the boards mentioned below do actually need
> > > bounce buffering?
> >
> > Unfortunately, I don't have any of the hardware to test and I havent
> > worked with that platform much.
> > That 'bb' variable and the fact that EFI needs bigger allocations is
> > why I ended up allocationg properly aligned memory from the EFI
> > subsystem. But as Mark pointed out, the cache flush is a no go for
> > now, so I'll drop this and see if I find time to rework the bounce
> > buffer logic overall
>
> There was quite a bit of discussion about all this in the context of
> my attempt to just add a message to warn the user[1]
>
> We might consider adding an event to reserve memory before relocation,
> along with a way to discover (in board_r) where the memory was
> allocated. That would make the solution more generic.

I am not sure what you are trying to solve here. The EFI bounce buffer
after the LMB patches can't overwrite memory, nor can it be
overwritten.

Thanks
/Ilias

>
> For the <4GB case we could perhaps add generic support for that in
> board_f, i.e. the ability to reserve a region for boards that need it.
>
> Regards,
> SImon
>
> [1] https://lore.kernel.org/u-boot/?q=%22Show+the+location+of+the+bounce+buffer+when+debugging%22