Re: [PATCH] MIPS: fix boot with DT passed via UHI

[Daniel Schwierzeck <daniel.schwierzeck@gmail.com>]

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Am 06.06.2017 um 21:16 schrieb Andrea Merello:
> commit 15f37e158892 ("MIPS: store the appended dtb address in a variable")
> seems to have introduced code that relies on delay slots after branch,
> however it seems that, since no directive ".set noreorder" is present, the
> AS already fills delay slots with NOPs.
> 
> This caused failure in assigning proper DT blob address to fw_passed_dtb
> variable, causing failure when booting passing DT via UHI; this has been
> seen on a Lantiq VR9 SoC (Fritzbox 3370) and u-boot as bootloader.
> 
> [    0.000000] Linux version 4.12.0-fritz+ (andrea@horizon) (gcc version 4.9.0 (GCC) ) #29 SMP Tue Jun 6 20:49:59 CEST 2017
> [    0.000000] SoC: xRX200 rev 1.2
> [    0.000000] bootconsole [early0] enabled
> [    0.000000] CPU0 revision is: 00019556 (MIPS 34Kc)
> [    0.000000] Determined physical RAM map:
> [    0.000000]  memory: 00696000 @ 00002000 (usable)
> [    0.000000]  memory: 00038000 @ 00698000 (usable after init)
> [    0.000000] Wasting 64 bytes for tracking 2 unused pages
> [    0.000000] Kernel panic - not syncing: No memory area to place a bootmap bitmap
> [    0.000000] Rebooting in 1 seconds..
> [    0.000000] Reboot failed -- System halted
> 
> This patch moves the instruction meant to be placed in the delay slot
> before the preceding BEQ instruction, while the delay slot will be
> filled with a NOP by the AS.
> 
> After this patch the kernel fetches the DR correctly
> 
> [    0.000000] Linux version 4.12.0-fritz+ (andrea@horizon) (gcc version 4.9.0 (GCC) ) #30 SMP
> Tue Jun 6 20:52:40 CEST 2017
> [    0.000000] SoC: xRX200 rev 1.2
> [    0.000000] bootconsole [early0] enabled
> [    0.000000] CPU0 revision is: 00019556 (MIPS 34Kc)
> [    0.000000] MIPS: machine is FRITZ3370 - Fritz!Box WLAN 3370
> [    0.000000] Determined physical RAM map:
> [    0.000000]  memory: 08000000 @ 00000000 (usable)
> [    0.000000] Detected 1 available secondary CPU(s)
> [    0.000000] Primary instruction cache 32kB, VIPT, 4-way, linesize 32 bytes.
> [    0.000000] Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
> [    0.000000] Zone ranges:
> [    0.000000]   Normal   [mem 0x0000000000000000-0x0000000007ffffff]
> [    0.000000] Movable zone start for each node
> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000000000000-0x0000000007ffffff]
> [    0.000000] Initmem setup node 0 [mem 0x0000000000000000-0x0000000007ffffff]
> [    0.000000] percpu: Embedded 15 pages/cpu @8110c000 s30176 r8192 d23072 u61440
> [    0.000000] Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 32512
> [    0.000000] Kernel command line: rootwait root=/dev/sda1 console=ttyLTQ0
> ...
> 
> Cc: linux-kernel@vger.kernel.org
> Cc: Jonas Gorski <jogo@openwrt.org>
> Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
> Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
> 
> diff --git a/arch/mips/kernel/head.S b/arch/mips/kernel/head.S
> index cf05220..d1bb506 100644
> --- a/arch/mips/kernel/head.S
> +++ b/arch/mips/kernel/head.S
> @@ -106,8 +106,8 @@ NESTED(kernel_entry, 16, sp)			# kernel entry point
>  	beq		t0, t1, dtb_found
>  #endif
>  	li		t1, -2
> -	beq		a0, t1, dtb_found
>  	move		t2, a1
> +	beq		a0, t1, dtb_found
>  
>  	li		t2, 0
>  dtb_found:
> 

The fix looks correct. Without ".set noreorder" one should not manually
put instructions in the delay slot. This should be left to the AS as an
option for optimization.

Acked-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

-- 
- Daniel


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