linux-kernel - Re: [PATCH] New config added to handle 64-bit systems with 32-bit DMA support

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Date: Wed, 19 Jun 2024 11:29:08 +0200
From: Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To: Alberto Secondi <albertosecondi@...il.com>
Cc: hch@....de, m.szyprowski@...sung.com, robin.murphy@....com, 
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	alberto.secondi@...nsula.com, Davide Salaris <davide.salaris@...nsula.com>
Subject: Re: [PATCH] New config added to handle 64-bit systems with 32-bit DMA support

Hi Alberto

On Wed, Jun 19, 2024 at 11:23 AM Michael Nazzareno Trimarchi
<michael@...rulasolutions.com> wrote:
>
> Hi Alberto
>
> On Wed, Jun 19, 2024 at 11:20 AM Alberto Secondi
> <albertosecondi@...il.com> wrote:
> >
> > From: Alberto Secondi <alberto.secondi@...nsula.com>
> >
> > The kernel assumes that 64-bit systems have 64-bit DMA support through
> > CONFIG_ARCH_DMA_ADDR_T_64BIT. This is not always true; for example, several
> > iMX8 systems (verified on iMX8MM and iMX8MP) have DMA with only 32-bit support.
> > This results in several drivers requesting DMA_BIT_MASK(64), which causes
> > malfunctions, particularly when systems have more than 3GB of DRAM (verified
> > with the lan743x driver and iMX8 systems with 4GB of DRAM). Therefore, a new
> > config ARCH_64BIT_HAS_DMA32_ONLY was added to manage 64-bit systems with 32-bit
> > DMA, which adjusts DMA_BIT_MASK(n) accordingly.
> >
> > Signed-off-by: Alberto Secondi <alberto.secondi@...nsula.com>
> > Co-developed-by: Davide Salaris <davide.salaris@...nsula.com>
> > ---
> >  include/linux/dma-mapping.h | 4 ++++
> >  kernel/dma/Kconfig          | 8 ++++++++
> >  2 files changed, 12 insertions(+)
> >
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index f693aafe221f..629220a777e3 100644
> > --- a/include/linux/dma-mapping.h
> > +++ b/include/linux/dma-mapping.h
> > @@ -74,7 +74,11 @@
> >   */
> >  #define DMA_MAPPING_ERROR              (~(dma_addr_t)0)
> >
> > +#ifdef CONFIG_ARCH_64BIT_HAS_DMA32_ONLY
> > +#define DMA_BIT_MASK(n)        (((n) > 32) ? ((1ULL<<(32))-1) : ((1ULL<<(n))-1))
> > +#else
> >  #define DMA_BIT_MASK(n)        (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> > +#endif
> >
>
> How can this fit configuration where you want to have one Kernel image
> for several arm64 machine?
>

I'm not an expert here but:

 @coherent_dma_mask: Like dma_mask, but for alloc_coherent mapping as not all
 *         hardware supports 64-bit addresses for consistent allocations
 *         such descriptors.

This looks more connected to what you are looking for.

Michael


> Michael
>
> >  #ifdef CONFIG_DMA_API_DEBUG
> >  void debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
> > diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> > index c06e56be0ca1..0a27eafed808 100644
> > --- a/kernel/dma/Kconfig
> > +++ b/kernel/dma/Kconfig
> > @@ -36,6 +36,14 @@ config NEED_DMA_MAP_STATE
> >  config ARCH_DMA_ADDR_T_64BIT
> >         def_bool 64BIT || PHYS_ADDR_T_64BIT
> >
> > +config ARCH_64BIT_HAS_DMA32_ONLY
> > +        bool "64bit System has DMA32 only"
> > +        depends on ARCH_DMA_ADDR_T_64BIT
> > +        default n
> > +       help
> > +         This enables forcing the maximum DMA_BIT_MASK to 32 bits for
> > +         64-bit systems that have DMA support limited to 32 bits.
> > +
> >  config ARCH_HAS_DMA_SET_MASK
> >         bool
> >
> > --
> > 2.34.1
> >
> >