irqchip/gic-v3-its: Limit number of per-device MSIs to the range the ITS supports

The ITS driver blindly assumes that EventIDs are in abundant supply, to the
point where it never checks how many the hardware actually supports.

It turns out that some pretty esoteric integrations make it so that only a
few bits are available, all the way down to a single bit.

Enforce the advertised limitation at the point of allocating the device
structure, and hope that the endpoint driver can deal with such limitation.

Fixes: 84a6a2e ("irqchip: GICv3: ITS: device allocation and configuration")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260206154816.3582887-1-maz@kernel.org

Author: Marc Zyngier

drivers/irqchip/irq-gic-v3-its.c

@@ -3475,6 +3475,7 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
 	int lpi_base;
 	int nr_lpis;
 	int nr_ites;
+	int id_bits;
 	int sz;
 
 	if (!its_alloc_device_table(its, dev_id))
@@ -3486,7 +3487,10 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
 	/*
 	 * Even if the device wants a single LPI, the ITT must be
 	 * sized as a power of two (and you need at least one bit...).
+	 * Also honor the ITS's own EID limit.
 	 */
+	id_bits = FIELD_GET(GITS_TYPER_IDBITS, its->typer) + 1;
+	nvecs = min_t(unsigned int, nvecs, BIT(id_bits));
 	nr_ites = max(2, nvecs);
 	sz = nr_ites * (FIELD_GET(GITS_TYPER_ITT_ENTRY_SIZE, its->typer) + 1);
 	sz = max(sz, ITS_ITT_ALIGN);

include/linux/irqchip/arm-gic-v3.h

@@ -394,6 +394,7 @@
 #define GITS_TYPER_VLPIS		(1UL << 1)
 #define GITS_TYPER_ITT_ENTRY_SIZE_SHIFT	4
 #define GITS_TYPER_ITT_ENTRY_SIZE	GENMASK_ULL(7, 4)
+#define GITS_TYPER_IDBITS		GENMASK_ULL(12, 8)
 #define GITS_TYPER_IDBITS_SHIFT		8
 #define GITS_TYPER_DEVBITS_SHIFT	13
 #define GITS_TYPER_DEVBITS		GENMASK_ULL(17, 13)