Re: [PATCH net-next 07/12] net: dsa: rzn1-a5psw: add statistics support

Andrew Lunn
2022-04-14T23:16:32Z

On Thu, Apr 14, 2022 at 02:22:45PM +0200, Clément Léger wrote:
> Add per-port statistics. This support requries to add a stat lock since
> statistics are stored in two 32 bits registers, the hi part one being
> global and latched when accessing the lo part.
>
> Signed-off-by: Clément Léger <clement.leger@bootlin.com>
> ---
> drivers/net/dsa/rzn1_a5psw.c | 101 +++++++++++++++++++++++++++++++++++
> drivers/net/dsa/rzn1_a5psw.h | 2 +
> 2 files changed, 103 insertions(+)
>
> diff --git a/drivers/net/dsa/rzn1_a5psw.c b/drivers/net/dsa/rzn1_a5psw.c
> index 5bee999f7050..7ab7d9054427 100644
> --- a/drivers/net/dsa/rzn1_a5psw.c
> +++ b/drivers/net/dsa/rzn1_a5psw.c
> @@ -16,6 +16,59 @@
>
> #include "rzn1_a5psw.h"
>
> +struct a5psw_stats {
> + u16 offset;
> + const char *name;
> +};
> +
> +#define STAT_DESC(_offset, _name) {.offset = _offset, .name = _name}
> +
> +static const struct a5psw_stats a5psw_stats[] = {
> + STAT_DESC(0x868, "aFrameTransmitted"),
> + STAT_DESC(0x86C, "aFrameReceived"),
> + STAT_DESC(0x8BC, "etherStatsetherStatsOversizePktsDropEvents"),

> +};

> +static void a5psw_get_strings(struct dsa_switch *ds, int port, u32 stringset,
> + uint8_t *data)
> +{
> + unsigned int u;
> +
> + if (stringset != ETH_SS_STATS)
> + return;
> +
> + for (u = 0; u < ARRAY_SIZE(a5psw_stats); u++) {
> + strncpy(data + u * ETH_GSTRING_LEN, a5psw_stats[u].name,
> + ETH_GSTRING_LEN);
> + }

The kernel strncpy() is like the user space one. It does not add a
NULL if the string is longer than ETH_GSTRING_LEN and it needs to
truncate. So there is a danger here.

What you find most drivers do is

struct a5psw_stats {
u16 offset;
const char name[ETH_GSTRING_LEN];
};

You should then get a compiler warning/error if you string is ever
longer than allowed. And use memcpy() rather than strcpy(), which is
faster anyway. But you do use up a bit more memory.

> +static void a5psw_get_ethtool_stats(struct dsa_switch *ds, int port,
> + uint64_t *data)
> +{
> + struct a5psw *a5psw = ds->priv;
> + u32 reg_lo, reg_hi;
> + unsigned int u;
> +
> + for (u = 0; u < ARRAY_SIZE(a5psw_stats); u++) {
> + /* A5PSW_STATS_HIWORD is global and thus, access must be
> + * exclusive
> + */

Could you explain that a bit more. The RTNL lock will prevent two
parallel calls to this function.

> + spin_lock(&a5psw->stats_lock);
> + reg_lo = a5psw_reg_readl(a5psw, a5psw_stats[u].offset +
> + A5PSW_PORT_OFFSET(port));
> + /* A5PSW_STATS_HIWORD is latched on stat read */
> + reg_hi = a5psw_reg_readl(a5psw, A5PSW_STATS_HIWORD);
> +
> + data[u] = ((u64)reg_hi << 32) | reg_lo;
> + spin_unlock(&a5psw->stats_lock);
> + }
> +}

Andrew