linux-kernel - [PATCH 1/3] riscv: Add support for kernel-mode FPU

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Date:   Tue, 21 Nov 2023 19:05:13 -0800
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Palmer Dabbelt <palmer@...belt.com>,
        Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        linux-riscv@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        David Airlie <airlied@...il.com>,
        Christian König <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Pan Xinhui <Xinhui.Pan@....com>,
        Daniel Vetter <daniel@...ll.ch>, amd-gfx@...ts.freedesktop.org,
        Samuel Holland <samuel.holland@...ive.com>
Subject: [PATCH 1/3] riscv: Add support for kernel-mode FPU

This is needed to support recent hardware in the amdgpu DRM driver. The
FPU code in that driver is not performance-critical, so only provide the
minimal support.

Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
---

 arch/riscv/include/asm/switch_to.h | 14 ++++++++++++++
 arch/riscv/kernel/process.c        |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index f90d8e42f3c7..4b15f1292fc4 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -63,6 +63,20 @@ static __always_inline bool has_fpu(void)
 	return riscv_has_extension_likely(RISCV_ISA_EXT_f) ||
 		riscv_has_extension_likely(RISCV_ISA_EXT_d);
 }
+
+static inline void kernel_fpu_begin(void)
+{
+	preempt_disable();
+	fstate_save(current, task_pt_regs(current));
+	csr_set(CSR_SSTATUS, SR_FS);
+}
+
+static inline void kernel_fpu_end(void)
+{
+	csr_clear(CSR_SSTATUS, SR_FS);
+	fstate_restore(current, task_pt_regs(current));
+	preempt_enable();
+}
 #else
 static __always_inline bool has_fpu(void) { return false; }
 #define fstate_save(task, regs) do { } while (0)
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 4f21d970a129..6a18bc709d1c 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -225,3 +225,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	p->thread.sp = (unsigned long)childregs; /* kernel sp */
 	return 0;
 }
+
+EXPORT_SYMBOL_GPL(__fstate_save);
+EXPORT_SYMBOL_GPL(__fstate_restore);
-- 
2.42.0