ARC: handle DSP presence in HW

When DSP extensions are present, some of the regular integer instructions
such as DIV, MACD etc are executed in the DSP unit with semantics alterable
by flags in DSP_CTRL aux register. This register is writable by userspace
and thus can potentially affect corresponding instructions in kernel code,
intentionally or otherwise. So safegaurd kernel by effectively disabling
DSP_CTRL upon bootup and every entry to kernel.

Do note that for this config we simply zero out the DSP_CTRL reg assuming
userspace doesn't really care about DSP. The next patch caters to the DSP
aware userspace where this reg is saved/restored upon kernel entry/exit.

Reviewed-by: Vineet Gupta <[e-mail]>
Signed-off-by: Eugeniy Paltsev <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/Kconfig

···
 	default y
 
 config ARC_HAS_ACCL_REGS
-	bool "Reg Pair ACCL:ACCH (FPU and/or MPY > 6)"
+	bool "Reg Pair ACCL:ACCH (FPU and/or MPY > 6 and/or DSP)"
 	default y
 	help
 	  Depending on the configuration, CPU can contain accumulator reg-pair
 	  (also referred to as r58:r59). These can also be used by gcc as GPR so
 	  kernel needs to save/restore per process
+
+config ARC_DSP_HANDLED
+	def_bool n
+
+choice
+	prompt "DSP support"
+	default ARC_DSP_NONE
+	help
+	  Depending on the configuration, CPU can contain DSP registers
+	  (ACC0_GLO, ACC0_GHI, DSP_BFLY0, DSP_CTRL, DSP_FFT_CTRL).
+	  Bellow is options describing how to handle these registers in
+	  interrupt entry / exit and in context switch.
+
+config ARC_DSP_NONE
+	bool "No DSP extension presence in HW"
+	help
+	  No DSP extension presence in HW
+
+config ARC_DSP_KERNEL
+	bool "DSP extension in HW, no support for userspace"
+	select ARC_HAS_ACCL_REGS
+	select ARC_DSP_HANDLED
+	help
+	  DSP extension presence in HW, no support for DSP-enabled userspace
+	  applications. We don't save / restore DSP registers and only do
+	  some minimal preparations so userspace won't be able to break kernel
+endchoice
 
 config ARC_IRQ_NO_AUTOSAVE
 	bool "Disable hardware autosave regfile on interrupts"

arch/arc/include/asm/arcregs.h

···
 #define ARC_AUX_DPFP_2H         0x304
 #define ARC_AUX_DPFP_STAT       0x305
 
+/*
+ * DSP-related registers
+ */
+#define ARC_AUX_DSP_BUILD	0x7A
+#define ARC_AUX_ACC0_LO		0x580
+#define ARC_AUX_ACC0_GLO	0x581
+#define ARC_AUX_ACC0_HI		0x582
+#define ARC_AUX_ACC0_GHI	0x583
+#define ARC_AUX_DSP_BFLY0	0x598
+#define ARC_AUX_DSP_CTRL	0x59F
+#define ARC_AUX_DSP_FFT_CTRL	0x59E
+
 #ifndef __ASSEMBLY__
 
 #include <soc/arc/aux.h>

arch/arc/include/asm/dsp-impl.h

···
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 Synopsys, Inc. (www.synopsys.com)
+ *
+ * Author: Eugeniy Paltsev <[e-mail]>
+ */
+#ifndef __ASM_ARC_DSP_IMPL_H
+#define __ASM_ARC_DSP_IMPL_H
+
+#define DSP_CTRL_DISABLED_ALL		0
+
+#ifdef __ASSEMBLY__
+
+/* clobbers r5 register */
+.macro DSP_EARLY_INIT
+	lr	r5, [ARC_AUX_DSP_BUILD]
+	bmsk	r5, r5, 7
+	breq    r5, 0, 1f
+	mov	r5, DSP_CTRL_DISABLED_ALL
+	sr	r5, [ARC_AUX_DSP_CTRL]
+1:
+.endm
+
+/* clobbers r10, r11 registers pair */
+.macro DSP_SAVE_REGFILE_IRQ
+#if defined(CONFIG_ARC_DSP_KERNEL)
+	/*
+	 * Drop any changes to DSP_CTRL made by userspace so userspace won't be
+	 * able to break kernel - reset it to DSP_CTRL_DISABLED_ALL value
+	 */
+	mov	r10, DSP_CTRL_DISABLED_ALL
+	sr	r10, [ARC_AUX_DSP_CTRL]
+#endif /* ARC_DSP_KERNEL */
+.endm
+
+#else /* __ASEMBLY__ */
+
+#include <asm/asserts.h>
+
+static inline bool dsp_exist(void)
+{
+	struct bcr_generic bcr;
+
+	READ_BCR(ARC_AUX_DSP_BUILD, bcr);
+	return !!bcr.ver;
+}
+
+static inline void dsp_config_check(void)
+{
+	CHK_OPT_STRICT(CONFIG_ARC_DSP_HANDLED, dsp_exist());
+}
+
+#endif /* __ASEMBLY__ */
+#endif /* __ASM_ARC_DSP_IMPL_H */

arch/arc/include/asm/entry-arcv2.h

···
 #define __ASM_ARC_ENTRY_ARCV2_H
 
 #include <asm/asm-offsets.h>
+#include <asm/dsp-impl.h>
 #include <asm/irqflags-arcv2.h>
 #include <asm/thread_info.h>	/* For THREAD_SIZE */
 
···
 	ST2	r58, r59, PT_r58
 #endif
 
+	/* clobbers r10, r11 registers pair */
+	DSP_SAVE_REGFILE_IRQ
 .endm
 
 /*------------------------------------------------------------------------*/

arch/arc/kernel/head.S

···
 #include <asm/entry.h>
 #include <asm/arcregs.h>
 #include <asm/cache.h>
+#include <asm/dsp-impl.h>
 #include <asm/irqflags.h>
 
 .macro CPU_EARLY_SETUP
···
 #endif
 	kflag	r5
 #endif
+	; Config DSP_CTRL properly, so kernel may use integer multiply,
+	; multiply-accumulate, and divide operations
+	DSP_EARLY_INIT
 .endm
 
 	.section .init.text, "ax",@progbits

arch/arc/kernel/setup.c

···
 #include <asm/unwind.h>
 #include <asm/mach_desc.h>
 #include <asm/smp.h>
+#include <asm/dsp-impl.h>
 
 #define FIX_PTR(x)  __asm__ __volatile__(";" : "+r"(x))
 
···
 		/* Accumulator Low:High pair (r58:59) present if DSP MPY or FPU */
 		present = cpu->extn_mpy.dsp | cpu->extn.fpu_sp | cpu->extn.fpu_dp;
 		CHK_OPT_STRICT(CONFIG_ARC_HAS_ACCL_REGS, present);
+
+		dsp_config_check();
 	}
 }