[PATCH] ARM: vfp: fix save and restore when running on pre-VFPv3 and CONFIG_VFPv3 set

[Måns Rullgård]

Russell King - ARM Linux <linux at arm.linux.org.uk> writes:

> On Sat, Oct 13, 2012 at 04:46:18AM +0000, Paul Walmsley wrote:
>> 
>> After commit 846a136881b8f73c1f74250bf6acfaa309cab1f2 ("ARM: vfp: fix
>> saving d16-d31 vfp registers on v6+ kernels"), the OMAP 2430SDP board
>> started crashing during boot with omap2plus_defconfig:
>> 
>> [    3.875122] mmcblk0: mmc0:e624 SD04G 3.69 GiB
>> [    3.915954]  mmcblk0: p1
>> [    4.086639] Internal error: Oops - undefined instruction: 0 [#1] SMP ARM
>> [    4.093719] Modules linked in:
>> [    4.096954] CPU: 0    Not tainted  (3.6.0-02232-g759e00b #570)
>> [    4.103149] PC is at vfp_reload_hw+0x1c/0x44
>> [    4.107666] LR is at __und_usr_fault_32+0x0/0x8
>> 
>> It turns out that the context save/restore fix unmasked a latent bug in 
>> commit 5aaf254409f8d58229107b59507a8235b715a960 ("ARM: 6203/1: Make VFPv3 
>> usable on ARMv6").  When CONFIG_VFPv3 is set, but the kernel is booted on 
>> a pre-VFPv3 core, the code attempts to save and restore the d16-d31 VFP 
>> registers.  These are only present on non-D16 VFPv3+, so the kernel dies 
>> with an undefined instruction exception.  The kernel didn't crash before 
>> commit 846a136 because the save and restore code only touched d0-d15, 
>> present on all VFP.
>> 
>> Fix to save and restore the d16-d31 registers only if they are
>> present.
>
> No.  VFPv3D16 HWCAP means that the VFP supports just 16 double registers
> - and it communicates this information to userspace.  If this bit is clear,
> and the VFP only has 16 double registers, then _that_ is a bug.

That bit will be clear on VFPv2 (because it's not v3), and this has only
16 D registers.  The high D registers are present if (vfpv3 && !vfpv3d16).
Pre-calculating this to avoid doing it in the context save/restore code
is probably a good idea.

-- 
Måns Rullgård
mans at mansr.com