Subject: Re: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()

From: Gerd Bayer <[email-masked]>
Date: Fri, 01 Aug 2025 13:30:27 +0200

On Fri, 2025-08-01 at 16:24 +0530, Manivannan Sadhasivam wrote:

<--- snip --->

> > > > > > The pci_bus_read_config() interface itself may have been a
> > > > > > mistake, can't the callers just use the underlying helpers
> > > > > > directly?
> > > > > > 
> > > > > 
> > > > > They can! Since the callers of this API is mostly the macros, we can easily
> > > > > implement the logic to call relevant accessors based on the requested size.
> > > > > 
> > > > > Hans, could you please respin the series based the feedback since the series is
> > > > > dropped for 6.17.
> > > > > 
> > > > 
> > > > Dear all,
> > > > 
> > > > I am once again deeply sorry for the problems that occurred in this series.
> > > > I only test pulling the ARM platform.
> > > > 
> > > > Thank you very much, Gerd, for reporting the problem.

no worries!

> > > > Thank you all for your discussions and suggestions for revision.
> > > > 
> > > > Hi Mani,
> > > > 
> > > > Geert provided a solution. My patch based on this is as follows. Please
> > > > check if there are any problems.
> > > > https://lore.kernel.org/linux-pci/CAMuHMdVwFeV46oCid_sMHjXfP+yyGTpBfs9t3uaa=[email-masked]/
> > > > 
> > > > Also, please ask Gerd to help test whether it works properly. Thank you very
> > > > much.
> > > > 

I found Geert's proposal intriguing for a quick resolution of the
issue. Yet, I have not tried that proposal, though.

Instead I spent some more cycles on Lukas' and Mani's question about
the value of the pci_bus_read_config() helper. So I changed
PCI_FIND_NEXT_CAP and PCI_FIND_NEXT_EXT_CAP to use size-aware versions
of read_cfg accessor functions like this:

diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index ac954584d991..9e2f75ede95f 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -109,17 +109,17 @@ int pci_bus_read_config(void *priv, unsigned int
devfn, int where, u32 size,
 ({                                                                   
\
        int __ttl = PCI_FIND_CAP_TTL;                                 
\
        u8 __id, __found_pos = 0;                                     
\
-       u32 __pos = (start);                                          
\
-       u32 __ent;                                                    
\
+       u8 __pos = (start);                                           
\
+       u16 __ent;                                                    
\
                                                                      
\
-       read_cfg(args, __pos, 1, &__pos);                             
\
+       read_cfg##_byte(args, __pos, &__pos);                         
\
                                                                      
\
        while (__ttl--) {                                             
\
                if (__pos < PCI_STD_HEADER_SIZEOF)                    
\
                        break;                                        
\
                                                                      
\
                __pos = ALIGN_DOWN(__pos, 4);                         
\
-               read_cfg(args, __pos, 2, &__ent);                     
\
+               read_cfg##_word(args, __pos, &__ent);                 
\
                                                                      
\
                __id = FIELD_GET(PCI_CAP_ID_MASK, __ent);             
\
                if (__id == 0xff)                                     
\
@@ -158,7 +158,7 @@ int pci_bus_read_config(void *priv, unsigned int
devfn, int where, u32 size,
                                                                      
\
        __ttl = (PCI_CFG_SPACE_EXP_SIZE - PCI_CFG_SPACE_SIZE) / 8;    
\
        while (__ttl-- > 0 && __pos >= PCI_CFG_SPACE_SIZE) {          
\
-               __ret = read_cfg(args, __pos, 4, &__header);          
\
+               __ret = read_cfg##_dword(args, __pos, &__header);     
\
                if (__ret != PCIBIOS_SUCCESSFUL)                      
\
                        break;                                        
\
                                                                      
\

This fixes the issue for s390's use-cases. With that
pci_bus_read_config() becomes unused - and could be removed in further
refinements.
                                                                      
However, this probably breaks your dwc and cdns use-cases. I think,
with the accessor functions for dwc and cadence changed to follow the
{_byte|_word|_dword} naming pattern they could be used straight out of
PCI_FIND_NEXT_{EXT_}CAP, too. Then, dw_pcie_read_cfg() and
cdns_pcie_read_cfg become obsolete as well.

Thoughts?