RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs

[ Upstream commit d82f32202e0df7bf40d4b67c8a4ff9cea32df4d9 ]

Before attempting to support the pre-ratification version of vector
found on older T-Head CPUs, disallow "v" in riscv,isa on these
platforms. The deprecated property has no clear way to communicate
the specific version of vector that is supported and much of the vendor
provided software puts "v" in the isa string. riscv,isa-extensions
should be used instead. This should not be too much of a burden for
these systems, as the vendor shipped devicetrees and firmware do not
work with a mainline kernel and will require updating.

We can limit this restriction to only ignore v in riscv,isa on CPUs
that report T-Head's vendor ID and a zero marchid. Newer T-Head CPUs
that support the ratified version of vector should report non-zero
marchid, according to Guo Ren [1].

Link: https://lore.kernel.org/linux-riscv/CAJF2gTRy5eK73=d6s7CVy9m9pB8p4rAoMHM3cZFwzg=AuF7TDA@mail.gmail.com/
Fixes: dc6667a4e7e3 ("riscv: Extending cpufeature.c to detect V-extension")
Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20240223-tidings-shabby-607f086cb4d7@spud
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index e12cd22755c78257bea44b95b813b860922ea4f8..e39a905aca2483ae18a1a6fb9c65b2c56578ce72 100644 (file)
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -21,6 +21,7 @@
 #include <asm/hwprobe.h>
 #include <asm/patch.h>
 #include <asm/processor.h>
+#include <asm/sbi.h>
 #include <asm/vector.h>
 
 #include "copy-unaligned.h"
@@ -396,6 +397,20 @@ static void __init riscv_fill_hwcap_from_isa_string(unsigned long *isa2hwcap)
                        set_bit(RISCV_ISA_EXT_ZIHPM, isainfo->isa);
                }
 
+               /*
+                * "V" in ISA strings is ambiguous in practice: it should mean
+                * just the standard V-1.0 but vendors aren't well behaved.
+                * Many vendors with T-Head CPU cores which implement the 0.7.1
+                * version of the vector specification put "v" into their DTs.
+                * CPU cores with the ratified spec will contain non-zero
+                * marchid.
+                */
+               if (acpi_disabled && riscv_cached_mvendorid(cpu) == THEAD_VENDOR_ID &&
+                   riscv_cached_marchid(cpu) == 0x0) {
+                       this_hwcap &= ~isa2hwcap[RISCV_ISA_EXT_v];
+                       clear_bit(RISCV_ISA_EXT_v, isainfo->isa);
+               }
+
                /*
                 * All "okay" hart should have same isa. Set HWCAP based on
                 * common capabilities of every "okay" hart, in case they don't