[PATCH] efi_loader: remove EFI_BOUNCE_BUFFER

[Simon Glass]

Hi Ilias,

On Thu, 27 Mar 2025 at 15:19, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> 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.

I am thinking of we can create a single implementation of the
bouncebuf logic which also works for EFI.

I think the two sane things to do are:
- restrict U-Boot to using memory below 4GB for platforms which have
the DMA limitation
- create (in board_f) a special region below 4GB for use with the
bouncebuf logic


>
> 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