On Tue, 2010-08-17 at 10:43 +0800, Jin Dongming wrote:
> (2010/08/17 10:37), Huang Ying wrote:
> > On Tue, 2010-08-17 at 08:56 +0800, Jin Dongming wrote:
> >> acpi_pre_map() is used for remapping the physical address of I/O, so
> >> it should be return NULL or remapped virtual address. The problem
> >> is whether I/O remapping is successful or not, the function returns
> >> NULL always.
> >
> > No. NULL is returned for error path only. Please check the code again.
>
> There three places used the local variable vaddr in acpi_pre_map() in next-tree.
> 1. 115 vaddr = __acpi_try_ioremap(paddr, size);
> 2. 122 vaddr = ioremap(pg_off, pg_sz);
> 3. 135 vaddr = __acpi_try_ioremap(paddr, size);
>
> Let's think about the following statement.
> Assumption: the physical address has never been remapped.
> Step:
> 1. vaddr == NULL
> Because the physical address is not registered in the acpi_iomaps,
> it should be returned NULL from __acpi_try_ioremap().
>
> 2. vaddr == the virtual address of the physical address.
> Here if ioremap is successful, the value of vaddr should be
> the virtual address returned from ioremap().
>
> 3. vaddr == NULL <== IMPORTANT
> Here it is because the physical address has not been registered
> in the acpi_iomaps yet, it still return NULL from __acpi_try_ioremap().
> So it is why vaddr == NULL, even if the physical address has never
> been remapped.
>
> Result: vaddr == NULL.
return vaddr + (paddr - pg_off), is not NULL for common cases.
> And if the vaddr is not NULL, it could not be added into acpi_iomaps.
> Codes in acpi_pre_map() is like following.
>
> 134 spin_lock_irqsave(&acpi_iomaps_lock, flags);
> 135 vaddr = __acpi_try_ioremap(paddr, size); <== the 3rd step
> 136 if (vaddr) {
> 137 spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
> 138 iounmap(map->vaddr);
> 139 kfree(map);
> 140 return vaddr;
> 141 }
> 142 list_add_tail_rcu(&map->list, &acpi_iomaps); <== add into acpi_iomaps.
> 143 spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
Oops, this is a bug, and your patch really fix it. But I think you
should change the patch description. NULL is not returned in almost all
cases. Because (paddr - pg_off) is not zero in common cases.
Best Regards,
Huang Ying
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