Re: [PATCH v2 6/6] MIPS: cm: Probe GCR address from DeviceTree

[Serge Semin]

Hi Jiaxun, Thomas

On Wed, Jun 12, 2024 at 11:08:58AM +0100, Jiaxun Yang wrote:
> Traditionally, CM GCR address can be probed from CP0_CMGCRBase.
> 
> However there are chips in wild that do have CM GCR but CP0_CMGCRBase
> is not available from CPU point of view. Thus we need to be able to
> probe GCR address from DeviceTree.
> 
> It is implemented as:
> - If only CP0_CMGCRBase present, trust CP0_CMGCRBase
> - If only mti,mips-cm node present, trust mti,mips-cm reg prop
> - If both present, remap address space to address specified in dt
> 
> Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
> ---
> v2: Fix build warning (test bot)
> ---
>  arch/mips/kernel/mips-cm.c | 61 +++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 55 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/mips/kernel/mips-cm.c b/arch/mips/kernel/mips-cm.c
> index dddc9428fe58..02afc795ba8a 100644
> --- a/arch/mips/kernel/mips-cm.c
> +++ b/arch/mips/kernel/mips-cm.c
> @@ -5,6 +5,8 @@
>   */
>  
>  #include <linux/errno.h>
> +#include <linux/of_address.h>
> +#include <linux/of_fdt.h>
>  #include <linux/percpu.h>
>  #include <linux/spinlock.h>
>  
> @@ -179,23 +181,70 @@ static char *cm3_causes[32] = {
>  static DEFINE_PER_CPU_ALIGNED(spinlock_t, cm_core_lock);
>  static DEFINE_PER_CPU_ALIGNED(unsigned long, cm_core_lock_flags);
>  
> +static int __init mips_cm_fdt_scan(unsigned long node, const char *uname,
> +		int depth, void *data)
> +{
> +	u64 addr;
> +	unsigned long *cmgcr = data;
> +
> +	if (!of_flat_dt_is_compatible(node, "mti,mips-cm"))
> +		return 0;
> +
> +	addr = of_flat_dt_translate_address(node);
> +	if (addr == OF_BAD_ADDR || addr >= ULONG_MAX)
> +		*cmgcr = 0;
> +	else
> +		*cmgcr = addr;
> +
> +	return 0;
> +}
> +
>  phys_addr_t __init __weak mips_cm_phys_base(void)
>  {
> -	unsigned long cmgcr;
> +	unsigned long gcr_reg = 0, gcr_dt = 0;
> +
> +	if (of_have_populated_dt()) {
> +		int err;
> +		struct resource res;
> +		struct device_node *cm_node;
> +
> +		cm_node = of_find_compatible_node(of_root, NULL, "mti,mips-cm");
> +		if (cm_node) {
> +			err = of_address_to_resource(cm_node, 0, &res);
> +			of_node_put(cm_node);
> +			if (!err)
> +				gcr_dt = res.start;
> +		}
> +	} else {
> +		of_scan_flat_dt(mips_cm_fdt_scan, &gcr_dt);
> +	}
>  
>  	/* Check the CMGCRBase register is implemented */
>  	if (!(read_c0_config() & MIPS_CONF_M))
> -		return 0;
> +		return gcr_dt;
>  
>  	if (!(read_c0_config2() & MIPS_CONF_M))
> -		return 0;
> +		return gcr_dt;
>  
>  	if (!(read_c0_config3() & MIPS_CONF3_CMGCR))
> -		return 0;
> +		return gcr_dt;
>  
>  	/* Read the address from CMGCRBase */
> -	cmgcr = read_c0_cmgcrbase();
> -	return (cmgcr & MIPS_CMGCRF_BASE) << (36 - 32);
> +	gcr_reg = read_c0_cmgcrbase();
> +	gcr_reg = (gcr_reg & MIPS_CMGCRF_BASE) << (36 - 32);
> +
> +	/* If no of node, return straight away */
> +	if (!gcr_dt)
> +		return gcr_reg;
> +
> +	/* If the CMGCRBase mismatches with dt, remap it */
> +	if (gcr_reg != gcr_dt) {
> +		pr_info("Remapping CMGCRBase from 0x%08lx to 0x%08lx\n",
> +			gcr_reg, gcr_dt);

> +		change_gcr_base(CM_GCR_BASE_GCRBASE, gcr_dt);

This causes the kernel boot-up procedure to crash/hang-up because the
CM GCR base address is supposed to be redefined by means of the
already mapped CM GCR address space by accessing the
CM_GCR_BASE_GCRBASE register:
change_gcr_base()
+-> read_gcr_base()
    +-> addr_gcr_base()
        +-> return mips_gcr_base + CM_GCR_BASE_GCRBASE

By the time of the change_gcr_base() call in mips_cm_phys_base(), the
mips_gcr_base variable hasn't been defined. So the IO operations
performed in the change_gcr_base() method would be accessing the
NULL-based memory space. That's why the kernel crash/hanging-up.

In order to fix this we have to first map the CM GCR block at the
default base-address, then update the CM GCR-base CSR and after that
remap the CM GCR-space.

Please also note, the GCR_BASE field might be RO. It depends on the
IP-core configuration. So it's possible that the CM_GCR_BASE_GCRBASE
field update won't work. Although that will be detected a bit later in
the mips_cm_probe() method by comparing the address returned from
mips_cm_phys_base() and retrieved from the CM GCR-base CSR.

-Serge(y)

> +	}
> +
> +	return gcr_dt;
>  }
>  
>  phys_addr_t __init __weak mips_cm_l2sync_phys_base(void)
> 
> -- 
> 2.43.0
>