Re: [PATCH] PCI: dwc: Modify the check about MSI DMA mask 32-bit

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: 이왕석 <wangseok.lee@samsung.com>
Date: Wed, 30 Mar 2022 12:52:03 +0900

> > --------- Original Message ---------
> > Sender : Alexander Lobakin <alexandr.lobakin@intel.com>
> > Date : 2022-03-28 23:34 (GMT+9)
> > Title : Re: [PATCH] PCI: dwc: Modify the check about MSI DMA mask 32-bit
> >
> > From: 이왕석 <wangseok.lee@samsung.com>
> > Date: Mon, 28 Mar 2022 11:30:09 +0900
> >
> >>  If dma_mask is more than 32 bits this will trigger an error occurs when
> >>  dma_map_single_attrs() is performed.
> >>  
> >>  dma_map_single_attrs() -> dma_map_page_attrs()->
> >>  error return in dma_direct_map_page().
> >>  
> >>  On ARTPEC-8, this fails with:
> >>  artpec8-pcie 17200000.pcie: DMA addr 0x0000000106b052c8+2 overflow
> >>  (mask ffffffff, bus limit 27fffffff)
> >
> > Isn't it a bug in the platform DMA code? dma_set_mask(32)
> > explicitly says that the system *must not* give DMA addresses wider
> > than 32 bits. If the system can't satisfy this requirement, then it
> > should return failure on dma_set_mask(32) -- this way you will only
> > get the corresponding warning, but there'll be no overflows (as the
> > mask will not be changed).
> > The idea of this call is to try to avoid getting 33+ bit mappings
> > so that PCI controllers which support only 32-bit masks could still
> > work correctly on the 64-bit systems. If the call fails, then this
> > message gets printed that you've been warned and it's your
> > responsibility to make sure that the controller won't get truncated
> > addresses. Having the call succeeded and then 33+ bit DMA addresses
> > is wrong.
> >
> > Please correct me if I'm wrong.
> >
> 
> Hello, Alexander Lobakin
> Thanks for your review.
> 
> You are right.
> My concern is that case of trying to use 33+bit dma mappings on 
> 64bit system.
> It is about the call sequence of the functions related to dma 
> setting, not the operation of the dma_set_mask() function.
> If dma_set_mask(33+) is performed before dw_pcie_host_init()
> for using 33+bit dma mapping, following error occurs 
> in dma_map_single_attrs()
> ex) DMA addr 0x0000000106b052c8+2 overflow 
>    (mask ffffffff, bus limit 27fffffff)
> dma_set_mask(33+) -> dw_pcie_host_init(): dma_set_mask(32) ->
> dma_map_single_attrs() -> 
> error return in dma_direct_map_page(): 
> because dma addr is 33+ but masking value is 32
> 
> So if the user has already set dma_mask to 33+ in order to use 33+,
> i suggested to modify dma_set_mask(32) not to be called.
> 
> Please let me know your opinion.

I'm not super familiar with the DMA internals, so adding Chris here,
maybe he'd like to comment, but anyway, the lower/arch layer must
not give the DMA addresses wider than the number of bits passed to
dma_set_mask() if that call returned 0.

> 
> Thank you.

--- 8< ---

> >>  -- 
> >>  2.9.5
> >
> > Thanks,
> > Al

Thanks,
Al