Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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@xxxxxxxxx
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